E30 Zone

My lpg'd 335i has developed an annoying humming sound. It originally only happened at about 60mph but you can hear it most of the time now. It's there irrespective of revs or even being in gear.

Any suggestions for what I should check as I can't see anything loose, broken or rubbing. The car has now done 219k BTW

Could be the diff, they start to hum when they are wearing out....

could it be some sort of vacuum pipe
that has split or come away from where it is supposed to be

If it is like a droning hum could also be a wheel bearing on the way out, particularly if the droning is getting louder/more noticable over time. Does the noise increase with road speed, ie if you knock it into neutral at 50 and coast is the noise still the same? Try jacking up the wheels one at a time and spinning them, if it is a wheel bearing you should be able to tell when you spin the wheel by hand (one wheel will feel a bit rougher than the rest, you'll probably be able to hear the bearing thats knackered if it is a bearing)

Cheers
Brian

The noise stays the same whether you're in gear or not (and at any revs). It's loudest when coasting (i.e. not on throttle or brakes) at 30 and 60 mph

I jacked the car up last night and the bearings all seem fine and I couldn't see any parts of the car about to collapse or fall off (which is a relief).

There is a little bit of oil on the back of the diff, so I guess this could be leaking. Had a look but I couldn't see how to top it up. Can anyone tell me how I top it up and what with (I'm guessing gearbox oil)?

Cheers