My car makes a sound that is very similar to rod knock between 900 and ~2500 rpm (after that its gone, or I can't hear it). I did some searching and found that I could isolate the sound (accessory or engine internal) by pulling the serpentine belt and starting the car.

I tried it, and the sound went away completely. I reinstalled the belt, and started the car and the sound is back, so that means one of the pulleys on the serp belt track is doing it. I have no idea how to check them all, and I can't seem to find threads with a related problem. Anyone have an idea of where to start looking? Are any of the accessories run by the serp belt prone to making a knocking sound when they're going bad?

I want to get this fixed before I start modding the car, as I may be able to get it under warranty (dealer already replaced the timing belt thinking that was the cause). Thanks.

 

get a stick have a friend take it up till you hear the knock then put the stick to each of the acc and then put your ear to the stick and see whitch one is bad

 

I'd start off by taking the belt off again and rotating the idlers and accessories by hand to see if you can feel any roughness. If that doesn't show anything then I would employ an engine stethoscope. A stethoscope, while a big word, is not expensive and it can quickly pinpoint any sounds.

A few words about using a stethoscope around a moving belt: be extremely careful - a belt that catches the probe on the stethoscope will ****** it up so quickly you cannot release your grip in time to avoid running your hand through the belt as well.