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      01-14-2021, 10:14 AM   #1
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Trying to figure out a vibration/noise

My 09 128i has developed a noise and vibration that I'm trying to track down. There are no stored codes and whatever is causing it doesn't seem to be affecting driveability. Car has 136k miles but has been well-maintained throughout its life.

The condition is throttle-dependent, and most noticeable at low RPM and large throttle opening. It seems more pronounced when the car is at operating temperature. It is a deep resonance, accompanied by a high-frequency vibration. From 40mph up and in gear, it's there in every throttle position except for just barely cracked open (basically holding RPM). The further away you get from that neutral throttle position, the louder it gets, especially when opening the throttle. I've also noticed that it spikes just as the clutch is being engaged (pedal released), as if the transfer of torque from the clutch being engaged is applying greater transient pressure on whatever's cause the problem before it levels out to a more constant state. I've definitively ruled out tires, as it remains the same with the snows mounted.

It doesn't sound like any gear noise I've ever heard, so I'm thinking it's either a bearing or the exhaust. Bearing seems more likely to me, as if it were the exhaust, why wouldn't it do it at lower speeds? It can be provoked at lower speeds by hard throttle, but normally it's not evident below ~40mph. There's no sign of it revving in neutral. If it is a bearing, I can only think of a few possible culprits: transmission output bearing, center support bearing, and diff input and output bearings.

Took it to my indy and we put it on the lift and ran it. The vibration is still present, but not as strong without the interaction with the road. He seems inclined to think it may be internal to the transmission, or possibly the clutch or flywheel, though he didn't hear anything definitive with a stethoscope. Nothing is visibly shaking, nor are there any signs of anything rubbing.

Any additional thoughts or analysis are most welcome. I need to get this stopped before it vibrates my brain to mush.
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      01-15-2021, 10:30 AM   #2
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Any exhaust or intake mods? Too difficult to capture on video?

Do you think it could be the throwout bearing?
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      01-15-2021, 12:06 PM   #3
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Very hard to capture on video, as it's mostly felt. Zero mods to the engine, drivetrain, and exhaust.

I'm looking into NVH analysis tools now.
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      01-18-2021, 04:02 PM   #4
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I know you said it's felt more than heard, but here's something I would still consider trying anyway: Download a free frequency spectrum analyzer type app (like Spectroid) to your phone, and see if you can get the vibration to show up. If it does, get it do so in at least two different gears. Record the gear, engine speed, vehicle speed, and vibration frequency each time. This would allow you to narrow down your list of potential culprits. It sounds like we can be pretty confident that it's not before the transmission output shaft/bearing, but this should also be able to tell you if it's before or after the differential.
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      01-18-2021, 08:14 PM   #5
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That's exactly the direction I'm headed. I've got a Treysit, but it's not the easiest to use in this application. I'm going to set up my phone and do some testing when time permits. I'll report back when I have something further.
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