imo your desription indicates loose belt: i had the same problem that would pop up only when i was pulling 4ucktarded downshifts (before i learned to double-clutch and match revs). the jolt of going up a thousand rpm had the belt squeeling like an entire pig farm. go back to the tensioner and make sure you are moving it out more.

*edit - if the problem becomes consistent even at idle, like this guy who is posting RIGHT NOW,
http:/forums.2gnt.comdcboard.php?az=show_topic&forum=4&topic_id=77782&mode=full

it is most likely the tensioner bearing. i would put maintenance issues like this in the general discussion in the future to get more hits.