It's most likely the stem bolts themselves. Realign, tighten, and you're golden. Make sure, if you haven't, to put some anti-seize compound on the threads.
Also, just for shits and giggles, check to make sure the top cap screw is tight - not crushingly tight, but on the tight side of snug. This holds the bearing set in place.
Bear in mind that, if you tighten the stem bolts before tightening the top cap, you're kind of defeating the purpose. Tighten the top cap first, then realign the stem and tighten that down.
Edited to say "oh, one of those ADJUSTABLE stems..." Okay, there should be either one or two screws that clamp down and hold the stem onto the top of the steerer tube. Those are the ones to tighten down.
Edited by Tai (12/08/06 10:08 PM)
Fortis est veritas