I have a 2002 GMC Sierra 2500HD. The main belt constantly squeaks, it did not start squeaking until after I had my water pump and heater hoses replaced. The shop has replaced the belt 8 times over the last 2 months, and the squeak goes away for a little bit, then comes back. The shop mentioned that a pulley might be going bad, so I asked them to check it out. They said it wasn't a pulley and put another belt on it. It has started squeaking again. Does anyone have any idea as to what would be causing this?
Very carefully hold a large crayon,yellow works best, like a lumber yard might use against the edge of the belt while the motor is running.If the squeel stops you know its an alignment problem.The wax in the crayon will lubricate the belt and stop the noise.Try one side then the other.If you are lucky the yellow will be left on the misaligned pulley and show whether it is pushed in or out.
I would suggest a new shop. It takes a special kind of incompetence to try the same thing 8 times and expect different results.
A squeal can be caused by misaligned pulleys, although I've never experienced it. Here is a link describing this on an older Sierra where the power steering pulley is misaligned. Maybe the same deal is possible your water pump pulley.
Another thing to look at is the automatic belt tensioner. Read about it here. Replacing these seems common enough that multiple youtube videos were the first hits when I googled "gmc sierra belt tensioner". Go figure.
All very good answers to this question, but the recent work brings up another possibility. Many belts will start to squeal if they come into contact with even one drop of coolant. The cooling system should be pressure tested to see if any of the hoses or the water pump were improperly installed. A slow leak would explain the 24 hours before the squeal returns.
Somebody posted a very thorough fix from GM with diagrams and everything, for a misaligned power steering pump pulley. The link is above.