I drive a 2002 F-150 (4.2L) with 150K miles. A noise similar to the belts slipping started up the other day but stopped when I drove the truck for a short distance. So I thought slipping belts was the problem but the noise started up again even after driving several miles. The noise only comes on intermittently and not all the time. In fact, it never comes on when a mechanic is looking at it so they think I am hearing things. I had the belt changed out and the mechanic said he thought the water pump was going out. So I had that changed out. He had the alternator checked out and it was good. The truck runs fine but the noise still comes and goes. It sounds like a high pitch vibration or like something is rubbing against something else but only periodically. Any ideas on what it could be?

 

I forgot to mention the sound happens idling and running both.

 

Get yourself an automotive stethoscope and Isolate the sound, or have someone that has one do it for you.