slik98z

i gotta 98 z28, when i picked it up from the dealership the idle seemed a bit high, its an m6 and when i start it up it stays around 1100 rpms then after about 5 minutes of warming up (its been chilly out in northern va as of late too around 30-45 degress) the rpms drop and stay steady at 900 rpms. i had a friend that works at a shop replace a belt cause i had a squeek that sounded like it was coming from the idler pulley, some silicone spray and a new belt fixed it up perfectly. he also tried to pull any codes but nothing came up. but i have kinda a high pitched sound, kinda like a squeek coming from what sounds to be the bottom drivers side of the motor up front. i've noticed that my lights flicker (even before i installed subs), the rpms stay nicely in place, no irratic idling problems or anything. only problem i've seen so far is decreased gas mileage but that is also partly due to some stubborn Gt's. any ideas what this could be?
thnx in advance
Travis