a creak is usually caused by two things rubbing together, that are not supposed to rub together. lubing the area where the creaking is occurring will make the creaking go away, but might not correct the root problem, and might make it worse (as the lube will facilitate the rubbing together). usually the only time i'll use grease to solve a creak is when it's coming from the bottom bracket, greasing the cup threads will usually get rid of it. and if there's a creak coming from the seat/seatclamp area. anyway...
a creak coming from the handlebars can be pretty serious, and usually comes from where the handlebars are clamped in the stem.
if your stem is 26.0mm, and your bars are 25.4mm, that will cause a creaking. also, if your stem is a single bolt aluminum style of questionable quality (SR, Sakae, etc), there could be a deformity in the inside of the clamp, which would cause it to not get enough surface to surface contact with the bars. get a honing tool and run it on the inside of the stem to get rid of the deformity, but don't over do it. if it's a large deformity, some rough sandpaper, or a round file will work best.
the low thunking sound could be caused by the quill shifting inside the steerer tube of the fork. my old bike used to do this. it's pretty normal, as the OD of the stem can't be EXACTLY the same size as the ID of the steerer tube, otherwise, it won't go in smoothly, or at all. what happens, is, when the quill is tightened, that is the only part of the stem shaft that's "gripping" the inside of the steerer tube, so the rest of the shaft is free to wiggle a bit in there. it'll slip under pressure, which is why you get the thunking sound.
it could also be that your headset is a little loose, or your crown race doesn't quite fit on the fork, or that your hub is a little loose...without seeing the bike, it's hard to say what it is for sure, but those are a couple of the less obvious causes.
Last edited by OneTinSloth; 04-06-06 at 10:16 PM.