My other truck (the beater) is a 97 F150, 4.2L V6 with 135k miles.

I parked it in the driveway Saturday with no problems.

Started it Sunday morning and it's got a severe metal-on-metal banging coming from the engine.

I pulled the valve covers tonight, thinking there was something broken in the valve train, but all valves are operating properly. I also pulled the serpentine belt to verify it wasn't any of the belt-driven accessories.

What else could it be ? It really sounds like a bad lifter...only louder. Very rhythmic and matched to the engine RPM speed. Any thoughts ?

 

There's the possibility you lost/spun a rod bearing and it wore the crank journal and/or bearing. In that case, your hearing the big end of the rod slapping around on the crank journal. This usually happens while the engine is running, but, that doesn't mean it couldn't happen at start up. Have you noticed a decrease in oil pressure or no oil pressure now that it's making the noise? Is there anything in the oil? Many times, if you drain the oil into a pan that you can see the entire surface of the oil, you can notice a greyish film-like swirl on top the surface of the oil. That would be "fine" bearing debris and may indicate you spun or lost a bearing. Is there water in the oil? Water will compromise the oil's ability to maintain the film of oil that the bearings ride on and destroy an engine that way. Of cource, if you drain the oil and chunks of metal come out, then you know you definately got a serious internal problem. In the end, it may be tough to determine the problem without pulling the engine and taking it apart. Further, it's real hard, for me anyways, to diagnose the problem without hearing it. I just know there aren't many things that can cause a metal to metal slapping sound and none of them are good. Did you do a compression test on each cylinder to see if they are all similar? If you have one that has no compression, or lower than the others, you will have narrowed it down to a cylinder(s). You can use a long metal screw driver or rod by placing one end on various places of the engine and the other end you grab like your making a fist and place your fist against your ear. Keep trying different places until you find the spot that it's loudest. The spot that it's loudest will at least give you a better idea of where to start looking for the problem. It's not exact, but, it may get you close.