goes my bike. It appears to happen when I'm not spinning with even force (i.e. pushing extra hard to get up a hill or something) and when I hit bumps. At first I thought fenders (because the shop replaced this bottom bracket less than 6 months ago but now I'm wondering if it could be the crank or bottom bracket and what I could do to remedy it. I own bike repair manuals and read Sheldon's site but I don't know how to definitively diagnose this and then how to approach the repair/adjustment. This clunking doesn't happen always just here and there but I imagine that it is a big problem...fixies aren't suppose to make much noise, right?

Hmmm. Chain tension maybe? If you're chain tension is a little too high it will tend to make a "popping" sound.

You might want to make a real close inspection of your frame. Have read in the catastrophic failure thread about guys unknowingly riding cracked frames hearing noises for a while prior to failure. Hope for you its just a creaking handlebar.

Even if the tension has been different at different times? i've adjusted the tension multiple times since owning this bike and it appears as though the clunking is consistent. What could I do with the bike up on the stand to test out some of these theories?

Is your rear wheel definitely in straight?

Loose BB?

whoa! could be a thousand things! but having said that, BB is a frequent culprit.
is everything tight and greased where appropriate?
a bit of grease on threads actually means you can do em up tighter sometimes. but they'll come undone if they're too loose. catch 22

lance

If it were an uneven wheel or bottom bracket wouldn't it be more frequent or happen every rotation or something?

Again, lots to check.
Frame flex, bb, chain tension, and my recent favorite the masterlink (mine rubs against the fender bolt but only under high torsion) took a few rides to figure that one out.
Enjoy