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Tapping noise when letting off throttle


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Recently noticed a concerning noise coming from my engine bay. Whenever I let off the throttle while driving, either to shift or slowing down I’ll hear two or three taps or high pitched pops in the engine bay. It sounds like if you’re clapping but only hitting your fingers together and more metallic and light? Weird analogy but it’s the closest thing I can think of. When I let off the gas there’ll be about a second delay before I hear *TAP*.... *TAP.... *tap* with each tap being right after the other. There’s nothing loose in the engine bay possibly hitting against the firewall, choke opens up and closes all the way, nothing on the carb is loose. The sound doesn’t happen when revving in neutral only under load. The sound is most prominent when the engine is cold and becomes much quieter or goes away when the engine is fully warm. My guess is that it might be backfire through the carb because it only happens when cold when the choke is on so the mixture is more rich but it seems too consistent for that to be it. I might try driving around without a hood and see if I can see what’s going on. I can’t remember the last thing I did to the truck before the sound started happening so I can’t trace it back to anything specific. Any ideas? Thanks.

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Have someone start the engine when it's cold. Being mindful of the fan, though you shouldn't be anywhere near it... quickly run your hands over the cold exhaust manafold paying particular attention to each runner from the head, where it is bolted to the intake and where it bolts to the exhaust down pipe. You may not be able to finish before the pipes warm up so it may take several tries. What you are looking for is puffs of exhaust gasses from a bad gasket or a crack in the manifold. Have a good look, it might even be visible.


Heat will expand the metal and close off the leak making it quieter when warmed up.




Another possibility is the mechanical heat riser spring is broken and the heat riser flapping. It's in the picture above just to the right center of the exhaust manifold, just inside the reverse C shaped pipe from the exhaust to the intake manifold. (ERG pipe) It's on that rod sticking out to the right and is a weight and a coil spring.



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Thanks, I'll check both of these. Also, how is the heat riser spring supposed to be set? I remember messing with it a bit when my exhaust manifold was off the car but forgot about it and put it back in without paying much attention to it

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There is a flapper gate that forces hot exhaust up under the intake to warm it when the engine is cold. This helps vaporize the fuel droplets and the engine runs better, and with less pollution when cold. The spring unwinds as it heats and the gate moves out of the way and the exhaust travels directly to the down pipe. I don't like this system as it is prone to rust from the heat, the spring rots away and it can stick in the open position and do nothing or the closed position and over heat the intake. Wire it so it can't move and if the noise goes then that was it.

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