Not sure if thats what it is, its the one right by the tensioner. Starts squeaking about 10min after the car is started, and if you spray it with armor all, it stops. What exactly are underdrive pulleys? How do they help? Should I just get them to fix the problem
I've been there too and it's annoying as all hell. But simply go to the parts store and buy one. The idler pulley they have is the same that is attached to the tensioner as the other one in the serpentine system. You just simply have to unbolt the tensioner to get to that one. Mine took like 10 minutes to swap, plus I got like a 2 year warranty on it.
Underdrive pulleys is just a set of smaller crank pulley, larger water pump pulley, and larger alternator pulley. AC, P/S, tensioner, and idler pulleys all remain unchanged.
I had the same problem a while back; I replaced both the idler and tensioner pulley, along with a new belt, and the problem went away and haven't had a problem since. Those pulleys turn over quite a bit and the internal bearings just wear out over time. Good news is that they are cheap (<$20). Got mine from Advance Auto Parts.