how do i fix it?

It may be that the belt is slipping around the pulley's or maybe one of the pulley's are bad and not the belt.

it is loose or bad belt could poss be a pulley bearing starting to fail you have to access the belt from under previa remove inspection cover then you can see belts and tell more about them if you dont have a lift you will need to jack it up its not bad once you can see them and always use jack stands

either tighten or replace the belt depending on how its worn.