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Discussion Starter #1
So I have noticed lately that whenever I first start out on a drive there is very distinct rotational humm/vibration that can be heard inside (top up)and felt in the gas pedal. It only occurs during on-throttle and not when coasting; in or out of gear. It comes and goes rhythmically/ cyclically (what I call rotational) as I accelerate. It does get somewhat faster as I increase speed but not in relation to the amount of RPMs from the engine (the engine revs faster than the humm/vibration increases). It does tend to go away the longer I drive it. When I first start off it does get pretty noticeable in the "feel" of my hands on the steering wheel. It only occurs when "on-throttle", so I'm not thinking of anything to do with wheel bearings, brake pads (I only say that because I just recently replaced pads/rotors) or tires.

If the work schedule and weather will cooperate I need to jack up the rear end and check for slack/backlash in the drive line.

So barring any catastrophic rear-end explosions has anybody noticed these symptoms prior to a rear-end going out?
 

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The only diffs that were known to be an issue were the LSD diffs in the early 2007 base models (due to GM assembling them without the proper diff fluid additives). With a 2008 Redline, diff failures weren't common.

The classic symptoms of that were a whining noise coming from between the seats down below the "waterfall" on the back panel when reversing or turning slowly in a parking lot at full steering lock.

Try to get a better idea of where the sound is emanating from. The diff sits right between the seats directly behind the passenger compartment so if it is the diff, that's where you'd be hearing it. Otherwise, it could be related to something else. If the noise/vibration is something you've only noticed since doing the pads and rotors, it could be brake related. You can double check that work too just to be on the safe side.
 

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Discussion Starter #3
Thanks!

It has been some time since doing the brake pads and I just recently started noticing this phenomenon. I also double checked the hub bolts at each wheel while I was there. All were good. I will double check the pads though.

It could be something to do with colder temps?? It does tend to go away (or at least becomes much less noticeable) after a few miles of driving. I'm just puzzled that it only does this when I am accelerating; not while coasting. Maybe something to do with mounts (motor, trans).I just can't correlate the rhythm/ cadence with something the engine is doing RPM wise. I can feel it on my foot via the gas pedal and sometimes in the steering wheel (not a shimmy just a vibration).

Maybe I just need to turn up that new Kenwood and wait till something breaks...haha
 

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Thanks!

It has been some time since doing the brake pads and I just recently started noticing this phenomenon. I also double checked the hub bolts at each wheel while I was there. All were good. I will double check the pads though.

It could be something to do with colder temps?? It does tend to go away (or at least becomes much less noticeable) after a few miles of driving. I'm just puzzled that it only does this when I am accelerating; not while coasting. Maybe something to do with mounts (motor, trans).I just can't correlate the rhythm/ cadence with something the engine is doing RPM wise. I can feel it on my foot via the gas pedal and sometimes in the steering wheel (not a shimmy just a vibration).

Maybe I just need to turn up that new Kenwood and wait till something breaks...haha
It COULD be an axle. They will make more noise when you accelerate than when you coast. However I don't think that has anything to do with temp changing it. I had it happen to me but that was more of a knocking type sound than humming.

What you describe though sounds like drivetrain related but after the trans since the sound changes with vehicle speed and not engine speed.
 

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Discussion Starter #6 (Edited)
Thanks guys!

OK, more info; it's definitely engine related and more than likely cold start/pre-heat cycle related. I started the cold engine and after a couple of seconds rev'ed it up to 2k RPMs and held it there. The humm/vibration was present just sitting still. I then rev'ed it up to 3k and the cadence quickened and grew more intense. Just to be clear this vibration isn't something earth shattering just not exactly normal. It all but goes away after the temps reach normal operating range. After my errands I repeated the 2k hold/ 3k hold and it was not there. I have no codes but I am wondering about the VVT components. I did have the water pump changed but that was many miles ago. I would be reluctant to suspect any of the hardware/chains/ tensioners but I can't seem to remember what happened to elff's sweet white car. I think it was something timing chain related. I can't afford a Alfa Romeo replacement like he did.


EDIT: In my defense I don't normally sit there and rev the engine so I only noticed it while driving.
 

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It may just be me, but I can't tell what is happening from the video. Can you describe it?
 

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Discussion Starter #9
In the video it becomes noticeable around 2500 rpms and it's a drone/humm that goes in and out in intensity. It gets louder and quicker by 3000 rpms. It produces a palpable vibration that I can feel on the throttle pedal and on the steering wheel. Performance is not affected.

It's sort of like a prop driven plane as it winds up the engine prior to take off. I used to listen(not by choice) to C130's rev to almost full throttle and the same humm comes to mind

P.S. I must remember to turn phone sideways next time!
 

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Discussion Starter #11
If it happens at the same rpm in different gears it is not the differential.
This is just sitting still. I erroneously suspected this to be a driving condition as it only happened when on-throttle so the title of this thread now is very misleading. I mentioned earlier in this thread I didn't try to rev the engine while parked (as I normally don't do that).
 

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In the video it becomes noticeable around 2500 rpms and it's a drone/humm that goes in and out in intensity. It gets louder and quicker by 3000 rpms. It produces a palpable vibration that I can feel on the throttle pedal and on the steering wheel. Performance is not affected.

It's sort of like a prop driven plane as it winds up the engine prior to take off. I used to listen(not by choice) to C130's rev to almost full throttle and the same humm comes to mind

P.S. I must remember to turn phone sideways next time!
I was thinking is sounded similar to a twin engine warbird like a P-38 or B-25. I'm not sure what would cause that but I would think it would be perhaps a balance issue.

Some questions, how many miles are on the car? Have you done any work on the engine lately like perhaps a water pump? Did anything happen or change around the time you started hearing this?
 

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Discussion Starter #13
I was thinking is sounded similar to a twin engine warbird like a P-38 or B-25. I'm not sure what would cause that but I would think it would be perhaps a balance issue.

Some questions, how many miles are on the car? Have you done any work on the engine lately like perhaps a water pump? Did anything happen or change around the time you started hearing this?
58k miles
Water pump changed over a year ago
No other changes to it except an RKSport rear spoiler..haha
Maybe a guide or tensioner on the VVT chain side of things????
 

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Well, remember you have a chain that drives your water pump. That also drives the two balance shafts in the engine. Hence why I asked about the water pump. I would suspect something happening with this chain to cause the kind of rhythmic pulse in that video. That or the balancer on the crank.
 

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Discussion Starter #15
Well, remember you have a chain that drives your water pump. That also drives the two balance shafts in the engine. Hence why I asked about the water pump. I would suspect something happening with this chain to cause the kind of rhythmic pulse in that video. That or the balancer on the crank.
That chain is what I am calling the Variable Valve Timing (VVT) chain (maybe mistakenly so). Wasn't sure exactly what it did. All I do know is if the timing chain breaks while the engine is running it would not be good!

Thanks for all of the help guys!

EDIT: I wonder how much of the chain can be seen with the water pump pulley cover off?
 

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That chain is what I am calling the Variable Valve Timing (VVT) chain (maybe mistakenly so). Wasn't sure exactly what it did. All I do know is if the timing chain breaks while the engine is running it would not be good!

Thanks for all of the help guys!

EDIT: I wonder how much of the chain can be seen with the water pump pulley cover off?
There are TWO chains in the front of the engine. One runs the two cam shafts and is technically your traditional "timing" chain. This is the one that would be the VVT chain. The other runs the water pump and the two balance shafts. Below is a picture of my new 2.4 engine (2.0 looks identical) and you can see the two chains.

The silver one is the outside chain and goes from the crankshaft (bottom) up to the two cam shafts in the head (top of the engine). The darker chain is on the inside of the timing chain and goes from the crankshaft, to the water pump sprocket (on the left) then around one balance shaft sprocket (to the immediate right and slightly below the water pump sprocket), over a guide to the other balance shaft (right side behind the black guide for the timing chain) and back to the crank.

It's this second chain that may be causing an issue. Note, the balance shafts have been removed in this engine and those balance shaft pulleys are there since you need that chain for the water pump so they may look a bit differen in an engine that still has the balance shafts but they are in the position the balance shaft sprockets would be in normally.

112053


If your VVT timing chain is off, valves get bent, pistons get broke, springs can snap...basically very, very, VERY bad things happen.

If the balance shaft / Water Pump chain is off a tooth, I'm not sure what happens other than you may get some strange vibrations and harmonics...kind of what you're getting.

Now when the water pump is changed if the sprocket moves, you get a metal on metal grinding sound I believe and that's because it screws up the tensioner allowing the chain to have slack and rub on things it shouldn't...at least that's my understanding of it. If it were just to be off in timing though, I don't think it would make the same noise since the chain would still have tension. Anyone feel free to correct me if I'm wrong here.

For your question about how much of the chain you see with the water pump cover off on the front of the engine, you basically can only see the water pump sprocket that you see in the above picture and not much else. You certainly wouldn't be able to see the timing marks on the balance shafts and rotate the engine to see if they line up with the appropriate marks on the balance shaft chain. You would have to take the whole front cover off for that.
 

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Discussion Starter #17
Thanks for the pic.
I remember the mechanic that changed the water pump removed the front cover. I know this because he charged me an extra $20 for having to remove the Venom Brace! He claimed it was easier to do the water pump this way instead of using the pulley holder tool. But it has been a few thousand miles since the water pump was changed. I do agree that this is a logical explanation just not looking forward to removing the front cover. At 58K miles I would figure if the front cover came off that both chains would be replaced so as to be done with it.
 

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