92' GMC Sierra Loud Squeeling From Engine

I have a 92' GMC Sierra 4.3 litre V6 with about 207,XXX recently when i start it up the engine will squeel extrmely loud and even when im driving untill a point where it will stop but then just start again at random times usually when i start it again. had my mechanic check the drive belt nut he said it is not the problem, could it be power steering pump? need some advice!