clunk from under
 

So while driving home on Friday I shifted from 1st to neutral as I was sitting in traffic and I hear a "clunk" from somewhere around the clutch area.... I put the car back in first and then back into neutral to hear a clunk again. So, I assume the rubber disk is coming apart.... But I am not slipping, having zero shifting problems, no grinding, etc... After I bring the car home and shut it down the clutch makes no noise. I looked into the inspection hole and the slave cylinder seems to be moving within spec... Is this the beginnings of clutch failure or something else? Any ideas?

thanks!