So we just picked up a 2000 GXE and I noticed when we got it home that the engine makes a pop pop pop noise! You can feel it when you put your hand on the intake.. It does it once every revolution.. It does not get louder when you rev it, you cant hear it then.. also cant hear it under acceleration.. I dont think its a rod knock.. what else could it be? It sounds hollow kind of..

Possible sticking valve at the lower rpm range?

Hmm.. it doesnt sound like it either.. i checked the pcv valve but thats not it.. its weird...