To test to see if the noise is from the belt,.. you can put just about anything on it... Oil, chain wax, baby powder, boogers,... Then listen to see if the noise changes...
... but anything you put on it, to fix the noise, will only fix it 'till it wears off...
To stop the noise,... the only way I could do it, was to bevel the belt sides with an angle grinder....
Grrr!....
... It works though...