RZT50 trans idler arm jumping

[Tarheeler]

when I crank the mower the idler arm on the trans belt starts jumping erratically
-I pulled the arm off and the pulley bearing is smooth and tight
- inspected the belt / looks OK
What am I missing? trans going out

 

[Tarheeler]

Found out it was a small chunk of dry wood wedged perfectly into the (R)trans pulley - very unlikely but possible:confused2: a little better light this am and i was able to see it