I just bought a 91 XLT. It has about 85000 miles. Ran great when I bought it but after a 2000 miles it has developed a knock/rattle. Sometimes on start up it sounds like a diesel for a few seconds then it becomes less noticable at idle. I can hear it in neutral or in drive. At 2000 rpm it seems to have a fairly continous rattle. Seems to be coming from the passenger side. Plugs look good. Hooked a vacum gage and its steady at 20 inches. Pulled the plug wires one at a time and it still rattles. The previous owner had the heads rebuilt about 12,000 miles ago because of a burnt valve. The only code in the computer is 22 for map sensor out of range (this is on the engine off test) Any ideas? Thanks.