My first question is this: With the belt off I spin the alternator pulley clockwise and there is NO noise. If I spin it counter clockwise it makes a little squeak. Is this normal? I don't hear any odd sounds when the engine is running by the alternator. The sound is worse with the a/c on but I'd imagine this might put more stress on the belt tensioner? I put a stethoscope on the top part of the tensioner and it makes a slight noise, and gets much louder with the a/c turned on! Almost sounds like something inside is rattling around. My a/c works just fine and I thought it might have been the idler pulley right next to the tensioner and I just replaced that and the noise is still present. Truck is a 98 f150 5.4L with 55k miles. Thanks!

 

First, there is a big difference between a squeak and a rattle. Second, by your post, it sounds like you answered your own question. Just from the information given, it would be just guesses to answer without being there and hearing the "noise" in person. JMO

 

It was the belt tensioner!

 

 

Yea, that's what I thought. Those SHOULD always be changed when the belt is changed. It's recommended in writing over @ the Gates site.