Ok, vehicle in question is a 118k mile 2000 Expedition EB. Upon start-up there is a loud mechanical rattle coming from under the truck. I've climbed underneath and tugged on every component I could get my hands on (heat sheild, cat, exhaust tubing, etc) but found nothing loose. The sound stops after 30sec to 1 min of running or if I turn the rear A/C off. I borrowed the guage set from Autozone and the pressure checked out fine.

Thanks
Greg