Mystery Noise: Differential or Wheel Bearings?

[dvallis]

So, after removing the rear seats from my E30 I can hear a very loud whine from the rear of the car. It sounds like this at 70 mph. Here's a frequency spectra of that same noise.



Looks like the offending sound is about 6.5 KHz. Filtering out all the road noise etc. and you are left with this.

Things I have determined:

1. Noise does not change when I press or lift the gas, no matter what gear
2. Noise does not change if I turn the steering wheel
3. Noise does not change if I step on the brakes
4. Seems to be 100% speed dependent. Higher the speed, louder and higher frequency the whine
5. Changed my LSD diff fluid with 75-W140 Valvoline synthetic. No change

Anyone out there with ideas?

Thanks

[HennaE30]

Mystery Noise: Differential or Wheel Bearings?

So it is speed dependent only?

Where in the back does it sound like it's from? One of the sides or in the middle? Also what kind of bushing did you use when you put the diff back in, anything stiffer than OEM rubber and you'll notice progressively more whine the stiffer you go.

Also check that you tightened all the bolts down, unlikely that it's that, but you never know.

[dvallis]

It's definitely speed dependent only. The sound seems to be coming from dead center behind the rear seat. Maybe a bit to the driver's side. And it's a freaking loud 6K high pitched whine at 70 MPH.

I changed the fluid with the diff in place. Didn't touch any of the mountings.

So weird.

[redlightpete]

Diffs are loud, hence the super heavy 'trunk tar'. Once the pinion bearings wear out they are really loud.

[pandaboo911]

I have no trunk tar, no rear seat, solid differential mount and an LSD with 260k miles and it doesn't make the slightest noise.


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[TobyB]

:

1. Noise does not change when I press or lift the gas, no matter what gear

That seems to rule out the diff gears, at least- a noisy ring and pinion will change tone
(spectral content, usually, not frequency) when you lift.
It makes sense- if the drive side of the gears are chewed up, when you lift,
you're now loading the coast side of the gears, and if you match revs
perfectly, there should be no load on them at all.

If it's a wheel bearing, it usually changes when you corner reasonably hard.

(mp3 sounds like you have a table saw in the trunk. Which IS consistent with diff...
maybe the side case bearings? The pinion bearings SHOULD change tone with loading.
Was the thing ever 'rebuilt'?)

t
all I got,

[bmwstudent]

more charts and graphs I can't understand please

[chadthestampede]

Mine makes almost this exact same noise. I just assumed it was the diff.

[dvallis]

It freaking does sound like a "Table saw in the trunk"! ;-)

No clue if it was ever rebuilt. I've owned several track cars and the diff never sounded like this.

[jake5]

Have you guys checked the inline bearing on the driveline?

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[varg]

It's not the wheel bearings, bad wheel bearings make it sound like you're in the cheap seats on an airliner. My diff whines audibly through my solid condor rear end bushings, and it sounds like the noise you hear in a subway/metro when it's accelerating.

[dvallis]

Where is the driveshaft bearing in relation to inside the car. I don't have a lift in my garage so it's hard to see under there.

[Seawolf]

If that's a pic of your driveshaft, it doesn't look right at all.

That drive shaft must be centered in the tunnel, one bolt isn't fully home and it looks like the bearing might be in contact with the body on one side - that will make a hell of a racket inside the car if it is.

[dvallis]

That's not my driveshaft. Just a picture I found online. Where are the tunnel bracket bolts with respect to the inside of the car?

[Todd Black 88]

I have the same noise.
100% speed dependent. I hear it at around 15 mph and up, and gets louder with speed but not with throttle on/off. Clutch in at any speed and it still makes it.

I assume it's my diff because everything else is new or rebuilt, from the engine back.