Constant Loud Drivetrain Drone/Whirring Noise While Moving

[bmwman91]

I noticed last night that I get a pretty good amount of constant whirring from the car when I am driving. It is most apparent when I am coasting in neutral since the engine isn't making as much noise. There is some speed dependence to how loud it is and as I get down under ~5MPH it goes away, but there is no vibration or anything to indicate something is out of balance since it is a constant whirring sound. It isn't a brake caliper being seized since the wheels seem to spin freely and I can coast just fine.

It seems to have gotten worse over the last couple of months. I have a 4.10 LSD (small case) in there with a lot of miles on it...are there known failure modes for those? Are there any good ways to test the diff and tranny in the garage to see if there is an abnormal amount of rotating resistance/friction? I am suspecting that something may be dragging somewhere in the driveline which is also resulting in at least part of my poor gas mileage. Yesterday I averaged an actual 26.5MPG (OBC read 33.6MPG...but I have a custom engine and different injectors) on a 250 mile drive where I behaved and averaged 66MPH, which seems pretty low even for a built 2.1L M42. My mileage has actually been declining steadily over the last 5 years or so, and despite replacing everything in the injection system, nothing has really made a difference.

I would try to record a video to capture the sound, but I think even though it is quite loud, it wouldn't come out very well.

[straight6pwr]

when you have a noisy diff, you can definitely tell its coming from behind you. if you take your rear seat, spare tire, and trunk carpet out of the car and drive it, you should notice the noise get even louder if its from the rear end. the noise associated with the diff will generally be much louder on deceleration (with the car in gear), and almost disappear on acceleration.

other culprits are wheel bearings, but every time i've had one going bad, it was fairly obvious as the noise would change noticeably when loading the suspension around a corner.

maybe get the rear wheels up in the air and run the car and see if you can have a helper pinpoint the sound's location.

[Wanganstyle]

168mm factory LSD units are known to fail on the left side output bearing support top cap @ the load point. At this point the entire unit is toast and really should be replaced with a 188mm LSD for reliability.

[bmwman91]

Thanks guys. I have been getting a fair amount of noise from the driveline on deceleration, but I assumed it was the 11lb flywheel causing the tranny to rattle a lot from lack of damping. I had the car up on stands last night and all of the wheels spun freely (or relatively free in the case of the rears).

Wanganstyle, The noise I hear in the car sounds a lot like loud brake drag on the rotors, but I am running rubber bushings and I doubt that such a noise would be audible in the car. Is that the same sort of noise that a failing small case diff will make? Is there any way to visually inspect for that without taking the output shaft off?

I have been planning on swapping in the entire driveline (tranny, driveshaft, diff, output shafts) from a 1997+ 318ti since it is more robust overall...maybe I should consider stepping those plans up a bit.

[Wanganstyle]

The cap fails in a way that unless the entire unit is broken down into pieces you won't easily know it's damaged.

Once the side caps holding output bearing are removed the fractured left side bearing support will just fall out (with output bearing still pressed on)