thump under middle of truck

I have a 2001 Suburban 5.7L with 70,000 miles. I have heard a thump sound since I have owned the truck come from under the vehicle about middle of the truck. Somewhere near the passenger side area. I hear it after the truck has run for awhile and the truck is sitting still with the engine running. I borrowed my brother-in-law's Silverado with a 5.7L engine and heard the same noise. Can someone tell me what this is?

However, I hear it after I have driven somewhere and then parked and turned off the engine, and it is often 30 sec - 1 minute or so after that. I don't know if it happens if I am just idling. Also, I wouldn't describe the noise as a thump, its more like the noise a coke can makes when you crush it. I would be very interested to hear what other people say about this.

its the baffle in the muffler. Temp related