Strange Engine Noise
 

OK, I know noise diagnosis is notoriously difficult without the actual sound. I sometimes get a strange low frequency humming buzzing, noise from the engine ('91 964 C2, Tiptronic) that is very rpm depended. It is most pronounced at idle and tends to go away at around 3000 rpm. It is there when the car is stationary, i.e not wheels, clutch, etc. dependent. It sounds like a fan, a pump, but then again... Any thoughts? I know it is vague. Thanks!

Is it more mechanical sounding our electrical sounding?

I bet the noise is not going away, you just can't hear it at the higher RPMs

Exhaust/cat shielding?
Power Steering pump?

More mechnical - almost like a fan running at lower speed. I did not think of the power steering pump. From the driver's seat it sound like the noise is coming from over my shoulder, i.e rear right... Thanks

Alternator bearing?

That was my first thought but apparently the alternator and bearing were replaced 2 years ago.... The search continues.

Start removing the belts one at a time. That's the only way you can isolate the source whether it's coming from the A/C, fan or alternator.