Audible humming when driving, wheel bearings or diff?

[alen]

stock 85 318i M10

I just replaced the driveshaft with a new flex joint and csb (old one was almost completely deteriorated). I also put in a replacement muffler (OEM) since the old one was shot and really loud.

Now that it's quiet in the cabin again I can start hearing all the other bad noises

One of them is what to me seems bad rear wheel bearings. There's noticeable humming coming from the back but only between speeds of 40-55 mph. The frequency also doesn't seem to be speed dependent. Now, all this noise sounds like bad wheel bearings, but I've also searched around a lot and noticed a mentioned of a possible bad diff.

Does anyone have a reference or been in a similar situation?

Thanks

[Sharino]

I had a similar noise years ago and I thought it was wheel bearings. Changed them and the noise was still there. I swapped diffs, and the noise was still there.

Problem ended up being the rear subframe bushing was worn out completely and transferring noise from the diff from the subframe to the body of the car.

Worth a check if you are looking under there.

[alen]

Thanks for the info. I think I know which bushing you're talking about. If so, I believe it looked ok. I'll snap a pic and post it to see if it's good or not.

[dude8383]

Easy way to check if your bearing is shot:

Take of the rear wheels and rotate the hub. If it doesn't rotate smoothly then it is highly likely that the bearing is done.

I had a similar humming coming from the rear...it would appear after 30mph and hum all the way to like 50 where the speed would just drown it out.

Ended up replacing the diff and nothing changed, so after inspecting my rear bearings in the manner above, I found that the right rear felt very grainy...as if sand was in there along with the grease.

[alen]

I checked the rear by rotating the hubs and they felt smooth and no noise.
One note on the noise/vibration when it appears at 40mph. It starts to drastically die off at 50mph but you can still hear that it's barely there. Also, it doesn't matter if I'm on the gas, in gear, or in neutral, it doesn't seem to affect the noise. Wouldn't this point out either wheel bearings or the subframe bushing?

Alen

[dude8383]

I checked the rear by rotating the hubs and they felt smooth and no noise.
One note on the noise/vibration when it appears at 40mph. It starts to drastically die off at 50mph but you can still hear that it's barely there. Also, it doesn't matter if I'm on the gas, in gear, or in neutral, it doesn't seem to affect the noise. Wouldn't this point out either wheel bearings or the subframe bushing?

Alen

Was there any play in the hub?

[alen]

No play in the hub.