I have a 91 C1500 5.0 with 73,000 miles on it and it has developed a squeak/chirp from the serpentine belt. It only happens when it's cold outside but when the weather is in the low 50's it has the squeak until it warms up. I have replaced the tensioner and idler pulleys as well as replaced the belt a couple times. I'm lost at what it might be. When I take the belt off and inspect the pulleys and spin them, there doesn't seem to be any issues with any of them.

 

I had a similar problem a few years ago and it turned out to be the wrong belt. The parts store gave me the wrong belt, it was a bit too long and this caused it to slip a little. The correct belt fixed the problem immediately.