Okay, well I've identified the problem and I thought I might post it just so nobody wastes their time thinking on it.
The thing (I really hate it when I forget what things are called.. Damned CRS) that bolts the exhaust pipe to the frame rusted through and the pipe is getting bounced against the gas tank. [insert great sigh of relief here]
I'm getting a welder on monday and I'll have it fixed then.
-Zen