Clunk when braking?

[ozymandias]

Hey all,

Background:

Recently acquired a Suburban (72 3/4 ton GMC w/ a 350 & 4spd manual) and have been slowly making it into a daily driver / work truck (with a lot of knowledge gained from posts here).

I've got quite a list of maintenance / upgrades to go through, but I've got a noise I can't pinpoint and want to get some feedback from everyone to see what the likely culprit is.

The problem:

The first time I step on the brakes, I hear a clunk that sounds like it's coming from the rear. The noise only happens once during a drive, but if I let it sit for a bit (anything over ~5-10 minutes), the noise will happen again (once) when I start driving and press the brakes.

What I've done:

Not much - mostly visual checks. I didn't see any leakage - especially from the brake drums or rear end, u-joints looked ok and didn't have too much play. I replaced the speedo cable and changed the transmission fluid.

Any ideas? All help is greatly appreciated!