98 sierra - squeeling engine

hi, i hope someone can help me, i have a 98 sierra and the engine is squeeling like a banshee.. i've change the alternator, waterpump, idler pulley and the tension pulley, along with the belt. it starts to happen after its run for a while, and it stops when i'm accelerating, but as soon as i let off the gas its starts again, or when its in park it really goes. any ideas?? thanks

clutch fan i would guess

Take off the serp. belt and spin the pullies. That should let you know where the problem is.

That loud I'd guess the alternator even though you've replaced it.

Hope this helps.

Check your oil pressure. You may have a clogged oil pump pickup screen.