I have "floated" sound boards a few times on SYSTEM 9/11 and DE pins to help determine the cause of the hum. So far I have found these causes of HUM (most common problems toward the top):

missing screws or missing GROUND (braided wire) connection in the backbox
bad connection on plugs
bad speaker
bad amplifier on sound board/MPU
bad caps on sound board/MPU
bad diodes/resistors on sound board
bad caps on power supply
bad potentiometer in cabinet or sound board
bad connector/open solder joints
low/high voltage on sound board
bad motor in the game somewhere
bad GI bulbs/sockets/feature lamp wiring (I know, a very strange one)

I use the "floating" trick but only after first checking all the above things.

I actually owned a Jokerz! long ago and my hum went away after I performed a playfield swap. I assumed that it was probably a bad connection on the playfield lamp matrix or GI somewhere.

You may want to check the resistance of the motor to see if it's still within spec...

You may also want to try to ground the frame of the motor to see if the hum goes away before you just assume it's bad.