It almost sounds like a pressure plate coming loose and starting to hit the bellhousing...(hope not)
Are you sure its coming from the tranny and not the engine? I hope it has nothing to do with the thrust bearings in the block since your relieving them when you depress the clutch.
Id check the tranny fluid level first @ the fill plug then drain it and look for shavings or glitter. Then id do the same to the engine. A magnetic drain plug can reveal some info too...