Broaden your search. I get that there have been loads of throw out bearing issues reported but it seems that it has become the go to recommendation and people are missing the small stuff by looking only for the big.
My guess (without being able to hear, see or feel things) is a loose heat shield or such that only vibrates when it is cold and shrunk up a little. As things warm up it expands just enough to not vibrate.
Like this:
http://www.ft86club.com/forums/showthread.php?t=97195