New to me '92 B230f NA Regina with 194k miles with unknown service history. Car runs fine, with no misfire or hesitation, good power, no CEL.

Engine makes a clackety noise, seemingly from the upper end only AT IDLE, noise goes away as the RPM's are increased over 2,000. The noise is definitely NOT a valve adjustment issue as it is a deeper tone, and it's NOT a rod knock, at least I don't think it's a rod knock as it is in the top end. It's not piston slap. It sounds pretty much like a loud DIESEL. It's not an accessory, water pump, etc. Spark plugs are good with no bad deposit patterns, and the compression is even 160 psi in all fours. Took the valve cover off and all seems fine there, nothing loose or excessively worn or scarred, not much tar or sludge indicating at least not a horribly neglected motor oil service wise... Would appreciate any guesses as to the possible source of the noise.