I like the line of thinking of the idler pulley. but check one other thing too - the darn plastic timing cover. Mine would warp out on the the car warmed up when I got the car and I couldn't figure out what was making this annoying noise. Turns out that the water pump pulley was rubbing the plastic and making a goofy noise. If I took the belt off the pulley didn't turn and the noise went away. talk about fun. There is a way to determine if the alternator is indeed making noise the noise. If you happen to have a large collection of old belts, find one that is just long enough to go from the crank pulley to the alternator. put it on (notice you are skipping the water pump.) and see if the noise is still there. If not, then the alternator is not to blame. To check the water pump, I have done the same thing using an old pair of nylons so everything else was out of the loop.