My 351 has a strange tapping sound from the passenger side of the engine. It is so fast that it almost sounds like a "buzz." As you gas it, it gets faster and louder almost sounding like a loud unpleasant hiss, and kind of just stops when the rpms get pretty high, like something "lets go" or cuts loose.

It appears to be loudest around the spark plugs.

Funniest thing of all, it is quiet and fine when the engine is cold. Valves? Lifters? I hope not.

 

Now that I think about it, it is almost more like a fast rattling noise. When You are going 35-40 it tends to happen most noticeably right before shifting gears (or could be the elevated RPMS cause it to seem louder then).

I thought transmission at one point but it does it after it is warmed up in park if you gas it. I even though the convertors were backed up because inside the truck it sounds like a hiss. Outside it is more like what I described.

 

Sorry to keep going but one last thing, when it is warmed up there is a noticeable power loss, and it hesitates to shift a little. When it is cold, it is quiet and fast.

 

Plugs torqued right?

 

loose torque converter bolt or bolts?

 

on the back side of the heads is a crossover tube(exhaust) for emissions, only 1 bolt per side secures it to the head. check those.

 

Sounds like the PCV valve problem I had. Check that too.

 

Dirty Fuel Injector Maybe !
Try some cleaner, it wouldn't hurt !

 

Maybe it is the PCV valve rattling? If it is that, its a good thing.

 

A cracked exhanust manifold will cause this on a small block. It will be quiet when cold and when the manifold heats up it will buzz or sound like a lifter is going when it is hot.