odd metallic rattling noise

I have a 2004 Prius with about 40,000 miles on it. I haven't driven it much in the mountains, but I recently started venturing to the mountains and have discovered a loud and disconcerting noise coming from the car after a long climb. The noise is a distinctly metallic clanging that occurs only when I let off the gas and coast. If I accelerate the noise goes away until I coast again. It sounds like I am dragging cans behind the car! 5-10 minutes after making a climb the noise goes away. I checked the tailpipe to see if it was loose but it wasn't. Any ideas?
Thanks,
Bob