EDIT

This post was edited by moviegeek on Wed, Jan 14, 15 at 21:59

Did you wrap the belt around the idler pulley correctly or is the tension too tight or too loose?
The drum makes a noticeable jerk and thud just before stopping when you turn it off...leading me to think it's the idler pulley or tension.
Or something isn't sitting right on the drum wheels or guides causing it to slightly put added resistance on the pulley system.
Like it's not turning as smooth as possible and idler pulley is flexing and springing back.

Here is a link that might be useful: Belt Replacement