Last year our arm would do this...
On nearly ever start up the robot would jump the drill motors powering our arm and this created a short grinding noise from the sprocket jumping the chain....
I think this only happened though when the IO was being powered from the RC with the tether?
We never really sought out a fix; we just laughed ever time it happened
(We considered the grinding noise to be the tigers ROAR!)
It probably has something to do with the firmware/software in the RC?