Does it make this noise when you have the foot off the cluth in gear, and out of gear??
if it does when you have it out of gear only it mostl likely is in the input or idler shaft of the tranny.
if it happens only when in gear it might be in the idler or output shaft.
If possible try spinning the output shaft of transmisison while in neutral. and see if it makes the same sound.
by the sound of this you could have a broken bearing race and this would have all the litler rollers "bouncing arround" but it is not at the point where metal to metal damage (metal shavings) has occured.
one way or the other you will have to tear it up to find out.