Mystery Vibration on Cold Engine

I got a '90 Cherokee, 4.0L and when I start it on a cold engine, I am getting a metalic vibration coming from the front of the engine. (sounds like a metal plate is sitting on top of the engine and is freely rattling) It sounds like something is loose, but I cant identify the problem. I suspect it could be p/s pump, alternator, compressor, or the fan clutch. The noise gets louder when I put a load on the engine, by either shifting into gear, running the compressor, or putting pressure on the steering wheel. After about 5 minutes, the noise stops completely. To replicate this probelm, I must wait till the engine has cooled off for hours, and is ice cold. Any advice would be greatly appreciated.