I have this squeaky noise somtimes at idle. I just changed the idler pulley only to be dissapointed.
Could it be the little ac pulleys? Thats what im going to change next....

 

I'm one step ahead of you. Replaced my tensioner pulley which was squeaking, waited for next morning, squeak was still there, regreased another idler pulley (Smooth one right side of engine and down a bit) and still squeaking. There is one more idler pulley I need to regrease and hopefully it will stop. Take your belt off and spin the pulleys individually and you will see. Just dont stop when you find one LOL.

 

Thanks...