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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?
 

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Heard a similar sound years ago in a Dodge, turned out to be the diaphram linkage in the power brake booster. The change of vacuum could be related to the linkage makeing sound. Pull out the vacuum line to the booster, seal it with tape and try to duplicate the sound. No sound, problem found. Might also have someone touch the firewall when the sound happens,they could tell where it comes from from expermiting.
 

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This is hard to tell from your description. This could even be like a backfire. Does this engine have an air or pollution pump? Many times these have one way ck. valves or diverter valves that go bad causing noise like this.
 
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A blown "doughnut" gasket between the exhaust manifold and pipe could make this sound, or a loose connection.
 
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They are right not much to go on but check the heat riser valve located on the right exhaust manifold, they normally become very lose just watch it as you snap the throttle. You do not have to replace it until it gets sloppy.
 

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I vote for the U-joint theory. Get under the truck and wrap your paws around the driveshaft and try to twist it back and forth....see if there is any play in it. If there is play, you'll need to instal new U-joints.
 
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