Join Date: Nov 2000
Location: New Orleans, LA
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It's not likely the stem. If the stem were that loose it would come out in your hands. As Mark says, it is the headset. It is a pretty easy adjustment. Where the quill enters the head tube there should be a large nut. Below that may be a big washer. Below that somewhere should be either another large nut or a knurled ring. If it will turn by hand, turn it until it feels like it contacts the bearings. It shouldn't be tight exactly, just not loose. Holding it firmly in position, tighten the big locking nut against the ring to lock the adjustment in place. Turn the bars. If it doesn't turn freely, loosen the lockring, back the knurled ring of a tad, retighten the locknut and try it again.
DISCLAIMER: The best approach and the proper thing might be a headset overhaul, including cleaning/replacing/repacking the bearings. This is just a quick fix that will get the clunk out.
If it ain't broke, mess with it anyway!