What causes this?

[Deltaskier]

On an 83 with a 350 & flowmasters, what makes it backfire every time I let off the gas? I know an exhaust leak could cause it, but the truck also rumbles & sometimes backfires when it's compression braking.

I hate it that every time I get on it, it backfires three or four times when I let off the gas & then makes that rumbling sound with some smaller backfires as it coasts.

Timimg? Exhaust leak? rich? Lean? I want to stop it, but I don't know where to start.