Fan Belt

I have a 1999 Chevy sliverado 4wd. Great condition, only 40k. After the truck Has ran for 10 15 min, and gets warmed up there is a terrible screaching/squealing noise from the fan belt. Only does this after it has been running for a while. You can shut it off, and start it right back up, and it will not do it for another min or 2. Have tried to wire brush it, to remove any film. Didn't help at all. Please help.

try just a touch of belt dressing, to see if it goes away. if almost sounds as though you have a bearing in something going bad. usually belts squeal at start up and go away.