Hello everyone and Thanks in advance for any help you might be able to give me. I apologize that this is lengthy, but I wanted to give as much information as possible.
I purchased a 2001 Vette about 2 years ago. It’s a Z51, A4 Coupe. Mainly stock other than a Donaldson Blacking, and BB Exhaust. Just over 50K on the clock. About 2 weeks after I purchased it, it started making a "rattling" noise of sorts that I have been chasing ever since.
The noise sounds a little like the regular LS1 chatter (just magnified), however, It’s only "loud" at 1300 to 1500 RPMS. Doesn't matter if cold, hot, etc.. Have done all the normal things to diagnose... 2 oil changes, took off all belts, etc..Noise is still there. Oil sample tested and came back normal. Oil pressure is totally normal for operating temps/rpms. When stethescoping engine while running, I can turn off each injector one at a time with my scanner.. No Change. At valve covers, coil packs, exh manifolds.. all seems normal and consistent from cylinder to cylinder. Valve covers off as well.. Nothing special in there.. Orange striped springs. All looks well. Next step…Balancer, timing cover off... There is some "slop" in timing chain. Something I would think I’d see on an 80K mile engine, but no smoking gun I guess. Replaced the harmonic balancer, timing chain, oil pump, AC Tensioner, AC Idler, Spark Plugs,Wires,Thermostat and a few other things I can’t think of right now.
Bottom line….none of that made any difference. Same exact noise, same exact RPM range. My next thought was lifters. My question on that was…if it was a lifter(s) why would it only do it at the 1300-1500 RPM? I’ve been around small black Chevys my whole life and I’ve never seen one act like this.
Runs out fine, no codes present and no loss of power or performance, but I’m honestly at the point of selling it, as it is bothering me that much. It’s almost embarrassing idling around a parking lot with it rattling like it is.
Any Help or ideas are truly appreciated. I will try to get a video or audio clip and try to get it posted.

 

How about the rocker arms? I've noticed some LS1's & LS6's sound like loud sewing machines at lower rpms due to the needle bearings in the factory rocker arms. You could try the trunnion upgrade.

Steve

 

I would put it on a lift with engine running and check for loose or rattling exhaust components. That can sound like engine noise.

 

It could be an AIR check valve or exhaust leak. Exhaust leaks can sound like bubble wrap popping. Mechanics stethoscopes are cheap and might help you pin point the noise.

It could be a loose bolt somewhere or foreign object rattling around. People drop things in the engine bay and they mysteriously disappear.

 

I am getting ready to install a Borla X-Pipe and Cat back system. I will make sure the converters are solid and that nothing looks out of place. I don't think this is an exhaust noise, but it could be.

 

I had a noise similar to what you describe, it was the inside of the cat converter had broken loose and was rattling around at certain RPM's.