Probably the clutch throw out bearing.  Or the cam belt tensioning
pully bearing.
Go to auto shop and get a mechanics stethoscope and poke aroung until
you find problem area.

listen carefully - I'll bet it's the fuel pump, or something to the rear of
hte car - I've heard this too, it comes and goes - never isolated the
problem, but I did put over 100K miles on the car with the sound happening
occasionally.  Could also be an idler wheel bearing, PS pump, etc - first
trick is to try to localize it

Paul had the same issue with my S2. I too had a new clutch and throw out
bearing and was mystified at the problem. Here is what it was...I had a
leaky PS rack, therefore the PS pump tended (with the release of fluid) to
moan. As you know the 944 PS system is variable, so as your RPM increased
the pump was under less stress and the noise went away. I replaced the rack
and the noise went away. Except...on spirited driving days I get a slight
moan if the fluid is slightly low. My old wrench figures the pump is getting
on in years, but should hold up for a few more. If mine starts the moan
heavily again...a new pump is not that expensive..