Might be that your chain is a little slack. I had this happen earlier tonight. A few clinks and clanks, and also a bit of rattle when I hit bumps and such. I stopped for a second, got off, shook the bike a bit, shuffled the cranks back and forth, and effectively, came to find that a slightly loose chain was the culprit. I probably didn't pull my rear wheel as far back as I could have when I changed the tires the other day. Which reminds me, I'd like to get myself a chain tensioner. Should make my life easier.

Anyways, cool story aside, once again, check your chain.