Vibration or buzzing noise
I’ve owned my BMW X5 for just under a year. It has covered 156,000 miles and has a good service history, although I’ve had to do lots of work to bring it up to scratch. The most recent irritation is a vibration/buzzing noise when stationary in traffic. It sounds like it may be coming from behind the dashboard, but is worse when in ‘Drive’ with my foot on the foot brake. It reduces a little if in ‘Park’, but a very light touch on the accelerator to raise the revs by no more than 50rpm causes it to disappear. I can also get it to disappear if the engine is raised slightly off its mounts by jacking slightly under the sump. The problem is at its worst when the engine is fully warmedup, so I’m not sure if the idle being raised slightly when cold is masking the problem?
The engine mounts in this vehicle have a comfort setting when idling which is vacuum-controlled and I have checked this out and not found a problem (ie, all lines hold a good vacuum, as do the mounts, and the ECU is energising the vacuum switch correctly and pulling the mounts to around 25in/hg at idle).
Do you think I’m on the right track in suspecting the mounts to be worn? Given the restricted access, they look a right pig to change, so I’d rather get the diagnosis right first. If it’s not the mounts, then it's something vibrating behind the dash, but I don’t fancy pulling that apart either. Mark Thompson As the noise is eliminated by raising the engine slightly, I believe you are on the right track and the noise is more likely to be coming from under the bonnet than behind the dash. Before going to the expense of replacing the engine mounts, I would check out the other components that may be altered by slightly raising the engine. One that immediately comes to mind is the power steering system hoses. The flexible hoses from the pump on the engine travel down to the steering rack and are supported by small flexible cotton reel-type mounts. I would check these thoroughly as a fractured mount could make exactly the noise you describe. I would also ensure that all engine covers are firmly fixed and that the exhaust system and heat shields are secure.