Its the belt or the idler/tensioner. I'd replace everything honestly if the car is older. At higher miles the bearings in the pullies just wear out and they aren't expensive on Rockauto or Amazon.
For the person who does it when the car is moving, probably balljoints.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.