Noise difficult to identify - help needed

[cirks]

I need some ideas as to what the cause of a weird noise coming from my car might be. At almost any speed, I'm getting a 'drumming' noise from (as far as I can tell) the front of the car. It sounds similar to a wheel-bearing beginning to wear but Saab have checked the car over (as have I) and there doesn't seem to be any movement in the wheel when jacked up. Saab also couldn't see anything obviously wrong. I've swapped tyres from back to front etc and the noise got slightly worse but I might be able to put that down to heavy wear on the 'new' front tyres. Brakes aren't binding and are in good condition.

What really confuses me is that the noise isn't a 'once per revolution' noise. ie. if you were counting the 'beats' you'd have 9 then 1 quiet then 9 noisy and 1 quiet etc.

Still makes the noise when taken out of gear and left to run under idle (so can't imagine it's engine related). Only other option I can think of would be gear-box but why the uneveness?

I know the source of car noises are difficult enough to identify when actually with the car let alone on a website but any (sensible )thoughts would be appreciated

Car is Saab 9-5 Aero Auto if that helps...

[Prof Beard]

Could it be one of the "idler wheels" on the auxilliary belt? - I'm not familiar with the 95 to that detail but 93s have long belts which go over multiple pulleys and and idlers. The bearing on these wear and can be noissy (and even collapse!)

[cirks]

Prof Beard said:

Could it be one of the "idler wheels" on the auxilliary belt? - I'm not familiar with the 95 to that detail but 93s have long belts which go over multiple pulleys and and idlers. The bearing on these wear and can be noissy (and even collapse!)

I have absolutely no idea but it's worth me checking. Thanks.

[Aero_saab]

Sounds silly, but check for odd wearing rear tyres.
Also rear trailing arm bushes can cause some odd wheel bearing type noises.

[cirks]

Aero_saab said:

Sounds silly, but check for odd wearing rear tyres.
Also rear trailing arm bushes can cause some odd wheel bearing type noises.

Doesn't sound silly but all tyres are wearing evenly. I can check the bushes though. Thanks