Loud clunk after lifting off accelerator

[n33k0]

Hello,

My 1972 C10 Stepside (350/TH350) has no shortage of annoying clunks and squeaks. The noise that is next on the to-do list is a loud clunk that only occurs when I lift my foot off the gas. Sometimes it only clunks once, sometimes two or three times in a row, but only when I lift my foot off the accelerator pedal. I have found that I can avoid it by gently lifting my foot off the pedal, but if I have to jump from the gas to the brake (getting cut off in traffic), something under the truck will most certainly clunk loudly. It only happens when the truck is in gear, as well; pressing the accelerator then quickly releasing it while in neutral does not cause any clunking noise. Any ideas what would cause this?