Ford F-150 1997 Rattling noise

When you mean rattle do you mean sparkknock or maybe bearing rattle? Narrow it down a little for me plz could be you catletic converter coming apart inside.(NOS) need more plz info if it is spark knock you have a timing problem.

Not sure what motor you have but i'll take a shot in the dark here and say it is a 4.6 or a 5.4 does the engine seem to idle a little fast and maybe stumble a little? If so lets try something here look for the line on the passanger side of engine towards the back find the PCV hose coming out of the valve cover follow that line up and behind the back side of the intake and see if that hose that is attached there is soft or has a hole in it if so replace it. Let me know what you find. A hole in that line is a vaccum leak and can cause spark knock. Do you have a check engine light on. How many miles is on the truck. And have you ever had the timing checked.