I have a 2004 5.3 with 120000 miles-starting making a tapping/knocking noise. I did a search and noise does not have same characteristics of anything I have read. It occurs when cold or hot, is not present at idle. It can be heard right before transmission shifts while driving or in park -starts tapping at 2200-2300 rpms up to around 3000. Engine runs perfectly, does not smoke or use oil. The only time you hear it while driving is right before shifting. I have taken vehicle to gm dealer and several local mechanics-opinions range from flex plate(confirmed it is not flex plate) cam, lifters, valve spring, cracked piston. Car runs fine and do not want to pull the whole engine apart to find it. Any thoughts would be greatly appreciated.

 

You didn't mention the year of the truck but I have found several things that can cause knocks sounding like they come from the engine. Mechanical fuel pumps with the push rod have been known to knock against the block. I once had a power steering pump that make a knocking sound but only when the AC was on. They ran off the same belt and of course, I found it after replacing the compressor. Since it can be heard while in park, use a long handled screwdriver and press the handle up against your ear and probe some places on the engine. The knock will come through the screwdriver loud and clear. Good luck.

 

It is a 2004, I ran it without the belt to eliminate alternator, powersteering, compressor,etc., mechanic was probing all around with his stethescope, hard to pinpoint. Really need some help with this one.

 

I had a motor do that once. When I idled it up and down around 3000 RPM you could hear it like a spun bearing. Turned out to be a wrist pin on a piston. Try that. The idle surge I mean.