I can't tell if the serpentine belt tensioner is bad. The tensioner is jumping back and forth as the belt runs around the pulley about a quarter to a half inch every half second as the car is running at idle. Is the tensioner supposed to move back and forth slightly like that as the car runs?

I tried to upload a video but it says not right file type🤷

 

not typical...i would replace both belt and tensioner. spin all the other pulleys while the belt is off...any with play or noisy, replace them also.

 

Thanks so much.🙏