When I start my 87 GT, The belts make a squealing sound. Before I did anything I wanted to know what it could be. Is it the belt or pully, I don't know. What can I do to find out what it is?

 

more than likely you just need to replace the belt.When you start it does it squeal then stop or does squeal all the time

 

If you going to replace the belt, buy the Goodyear Gatorback belts. I had issues with squeaky belts in the past with my 2 mustangs, and these belts resolved that issue. The teeth on the belts only go about halfway into the pulley grooves, which I believe helps avoid the squeaks.

 

either the belt is glazed or tensioner is weak, if tensioner is okay go for a gatorback belt.