Harmonic noise at high speed??

A

audihere

Testfahrer
Registriert
13 Oktober 2020
Wagen
BMW Z3 M coupé
I get a harmonic high pitched hum (pretty loud) on the freeway at 70+ MPH (faster is louder) when I keep the accelerator pedal in a position to maintain speed (not accerlerating, not let off pedal to decelerate). It's like the drive train is in equilibrium and something is loose. Because accelerating tightens it up and using engine decelerating tightens it up in the other direction and the noise goes away.

I've tried changing out the diff oil and machining E36 Rougue diff cover a little so it doesn't touch Randy Forbes pendant mounts.

Going to replace rear wheel bearings and overhaul axles next.

Anyone have any advice?

Thanks!
 
audihere schrieb:
I get a harmonic high pitched hum (pretty loud) on the freeway at 70+ MPH (faster is louder) when I keep the accelerator pedal in a position to maintain speed (not accerlerating, not let off pedal to decelerate). It's like the drive train is in equilibrium and something is loose. Because accelerating tightens it up and using engine decelerating tightens it up in the other direction and the noise goes away.

I've tried changing out the diff oil and machining E36 Rougue diff cover a little so it doesn't touch Randy Forbes pendant mounts.

Going to replace rear wheel bearings and overhaul axles next.

Anyone have any advice?

Thanks!
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Do you check the cardan shaft center bearing?
 
Sven.zet schrieb:
Do you check the cardan shaft center bearing?
Zum Vergrößern anklicken....

I haven't looked into that bearing yet, mainly because I found a long thread on another forum that had a guy with similar situation to mine and changed out the drive shaft bearing only to find it didn't help at all.

But with that said, it could the next thing for me to look into. Common sense tells me that could be causing the noise.

It just seems like I'm working my way towards the differential which could also be causing it, but hoping to find something cheaper to fix first! :)
 
I'm being told it could be directly related to when someone disassembled and reassembled the diff. Either the gears are not meshed correctly or there's an issue with the torque on the pinion nut. Sound plausible?
 
In my opinion, a hum indicates a defective wheel bearing, but this should be noticed first when driving curves. A defective cardan center bearing is more likely to cause vibrations at around 140 km / h. Defective drive shafts are similar to unbalanced wheels. I've never had a defective differential, so I can't comment on it.
 
I just had all four wheel bearings replaced when I went back to stock brakes and new dust shields. The noise did not change after new bearings installed.

The noise ONLY happens when I'm going over 65 MPH and I place the throttle into a position that I am not accelerating and I am not decelerating. At that point, the car will go into a harmonic tuning fork noise. At 80 MPH, the noise is much louder. If I let off, or press the accelerator pedal a little, the noise goes away completely.
 
Are there any vibrations? Otherwise, I'll type in your description on the exhaust system. Maybe it is built in taut.
 
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