Hi,
Could it be the clutch release spring/bearing? I have a droning noise on my car which is more noticeable when driven hard. When the droning occurs, I find taking the foot of the go pedal and depressing the clutch fully and re-engaging the clutch eliminates the drone. I've been told that this is common on 944 and replacing the clutch bearing doesn't always eliminate the problem. But you'd have to replace the clutch to find that out I'd guess- hence why I've learned to live with it 'till the clutch needs doing.
This may not be the same problem you have though.
Ben
