1998 jimmy vortec weird tapping

a guy i work with has a 1998 jimmy with the 4.3 vortec, i listened to the motor while it was runnin, and it seems to run fine with just a weird tap, sounds kinda like metal on plastic, however the serpentine belt was taken off and the truck was run makin the same noise, from what he said there was no power loss or anything, but he is tryin to take it easy on it, what i was wonderin is if this is a common problem or if anyone has run into this before and knows what it is, please let me know if anyone has an idea

Where is the noise coming from? It could be a lifter.

sounds like maybe top end, but i guess he took it to the shop and they said something to the effect that a bottom end bearing slipped, but i think they were blowin smoke up his ass, but i think its either a lifter, a bad valve seat or possibly a broken valve spring, but he said the noise started right after he had work done to his intake manifold, i think possibly an upper radiator hose or something with the injectors, im not totally sure what he had done, but it was something to the upper intake manifold. so im am really kinda stumpped, but this weekend or next weekend hopefully i can get it in my friends garage and get a good look at it, but if anyone has any ideas, please let me know

Maybe the fan shroud is hitting the fan??? Mine did that and I had to use zipties to hold it down so the fan would stop hitting it...thats what I get for climbing into the engine compartment lol.

no, its not the fan shroud, when he took it in the shop they took off the serpentine belt and it still made the same sound, it could be described as a heavy tap, but when i hit the throttle a little, it went away for a couple of seconds just to return

Sounds to me like a sticky valve lifter. I have been able to unstick an unruley lifter with that "Marvel Mystery Oil" before. Sounds kinda dangerous to put into a motor, but it really did work for me.

hey thanks, ill have him try that and ill let you know

The shop that said bottom end bearing is probably right. The sound you hear is piston skirt slapping cyl wall because crankshaft is runnuning on bad main bearings. Very common for this to happen right after intake manifold work. Improper torqing of the intake manifold on 1996 and up will distort the block, causing main bearing failure. Search this and the Blazer & S-10 forums for more info.