I have a '76 Chevy 3/4 ton. It's got the 4 barrel carb and full time 4 wheel drive. Lately, when I am driving along (usually in 4th gear) and then I suddenly and completely let off the gas, I hear a slapping sound coming from the engine-it sounds like it's coming from just in front of the cab. If I let the gas off very slowly, I don't hear it so much. It's almost a plastic sounding noise (like a rapping or clapping sound) as opposed to a metal grinding or knocking. Any ideas of what that could be?