I have a 97 F-150 and I am the original owner. The truck has 186,000 on it and has started to make a funny noise. The engine was replaced BTW at around 100,000 by ford becasue of bad lifters. So whenever I peg the truck in third or fourth gear there is a knocking noise I hear when my window is down. It is pretty noticeable. It only occurs from about 3500 rpms until 4000 then clears out. It also gets worse when I am towing my enclosed trailer. What could it be? I also pulled the truck into a car wash the other night and tried reving the engine fast and slow to see if I would hear it but it never occured. Truck runs like a top otherwise, but the noise onlly occurs under load with now power loss...

 

You are probably hearing rocker arm rattle. Might want to look back on the paperwork for the old engine because you don't have lifters.

 

btw its a 4.2l. Well if its the rockers is this going to cause damage, or is it safe to keep driving? I need to pull my trailer this weekend about 400 miles round trip. Is this going to ruin my engine?

 

I have done some reading online about this tonight. Could this be Rod knock I am hearing?

 

if its a 4.2 its a ohv engine. if it was rod knock it would be doing it all the time i would think. we have a 2005 gmc truck at work with the vortec 6.0 that does the same exact thing. only does it under load. did it from the first day we had it.i still have no idea what it is. got 20k on her now and she runs great so... my noise sounds more like a marble in the muffler type noise. after i get off the gas it goes away.

 

That is exactly how it sounds. Like a marble comming from under the truck. I figured if it was internal it would do it all the time too. Oh well man thanks for the input..