I have an engine rattle on the right side of the engine. I thought it was a vibration rattle, but I'm not so sure. It is loud inside the car but not very noticeable outside. It happens more in cold weather and when the engine is cold. The noise has been there for about 10 years and I can't find it. Has anyone else had the same thing? Is this a common Z28 trait?

 

have you checked your motor mount? does the engine "rotate" when you rev it? (more than usuall)?

 

Sounds like the EFE valve, aka heaty riser valve; which is a sort of a "spacer" looking thing where the Y-pipe joins to the right exhaust manifold, which has a vacuum-operated valve in it that forces some of the right side exhaust to go through the right head, the intake, and the left head, into the left exh man; to help warm up the intake faster for quicker cold-start driveability.