i've occasionaly encountered the same probrem. Mine was sort of vibration sounded like that comes from right behind center A/C vent. when it does, its really freakin' loud and annoying.
Turned out the my quadrant was vibrating. as soon as you toutch quadrant with tip of your feet, it stopped. Never figured out why.
then i changed clutch. Along with TOB and everything else. Now vibration is pretty much there to stay. same sound, and same location. Only this time, i've noticed that vibration corresponds with TOB sqweeling. From that, I have concluded that noise is something/somehow caused by TOB, and coming up through clutch cable. And to prove this, when vibration and TOB sqweel was there, i wiggled on clutch cable from engine bay. and both stopped immidiately(as long as i kept wiggling cable). can't imagine what it might be tho. If it happened everysingle time, i might have thought may be my pressure plate finger is bent, flywheel is not bolted on straight, clutch folk is dmg'ed. But as freaquent as it happens, its not everytime thing. So from that, i think its not from any i've just mentioned. Mostlikely something to do with tob being bad(like m SPEC one was), causing some sort of resonace, which traveling up through clucth cable, then vibrating clutch pedal assembry.
this is how far i figured out. Now as far as fix goes... i'm still trying to figure it out.
Hope this gives you some idea what you might be dealing with