What makes a serpentine belt squeak?

[94TURBO5 O]

I just bypassed my PS along with the AC and smog with a 56" belt and its snug but not super tight like my other one was. Will it sqeak on start up b/c it's too loose or snug? I wanna take this one back before I wear off the lettering on the belt and it looks real used.
Thanks!

 

[SN95Oh]

If the belt is too loose it will squeak. It may also squeak if get on it. So I would take it back and get one like 1/2-1 1/2 shorter depending on how much play you have.