I have a 2001 F150 with 136k miles. There is nothing wrong with the way it runs, but I can hear a loud clatter noise that only seems to occur in a fairly small range around 1,000 RPM. The noise is louder when the truck is moving, as opposed to just revving the engine in park. The noise seems to be coming from the front of the engine. I just replaced idler pulley, alternator, belt and belt tensioner. Tried running the engine with the belt off and I still have the same noise at 1,000 RPM. Mechanics want to start taking the engine apart to find out what it is. Not sure if I should just keep driving the truck like this or take it into the shop for costly diagnosis and almost certainly costlier fix.

 

With that many miles on your engine, I would venture to guess that it's the timing chain guides. The guides act like idler pulleys and keep tension on the timing chains. They could be worn out and the noise you hear are the chains themselves. If the guides break, it could be costly as the eng would probably need to be lifted to remove the pan to remove the broken debris.
Mike

 

The above statement is right. Could be the guides are worn, a weak chain tensioner, or the chains themselves. I would recommend getting it repaired asap if it is making that much noise. You will be better of doing it now then puttin a motor in it because a chain jumped. When and if you do have it fixed make sure whoever is doing the work to check the crankshaft exiter ring that is right behind the timing cover on the crank when they take it apart. Most likely from the chains being loose and slapping around in there it has been getting eaten up.