Originally Posted by layback40

First check oil in trans. then;
1/ While the car is stationary engine running, clutch down, in gear, headlights on, rear window heater on (if it has one), anything else electrical going. Rev the motor up to at least 2,500rpm. If its the belt, it should do it under these conditions. Repeat with car in neutral foot off clutch. You didnt rev the car enough in the audio.

2/With nothing electrical on, in neutral, rev motor, while revving motor any of the sound? Try pressing clutch down while in neutral and revving. Any of the sound?

3/ While driving down the road & the sound there, put car in neutral, does sound go away?
4/ While driving down the road with the noise, put foot on clutch, does noise change pitch or go away?
Please post the answers to these questions & we will have a better idea of the cause.
I am leaning to the thrust bearing or drive shaft bearing or even a stick jammed up under the car, still could be the alt belt though.