Neil,

Chock the wheels on level ground and crawl under your bus.  Then get someone to operate the clutch pedal to allow you to isolate the noise.  It could be coming from the slave cylinder as the plunger operates, or it could be coming from the cross-bar in the tranny that moves the throw-out bearing back and forth.  Or it could be coming from the throw-out bearing mechanism itself.

It's either that or somethin' else ;)

Good luck,

Warren L.
88 Westy "Billy Bones"