There's not much that can be repaired inside them - I tried repairing mine, but in the end bought a new one off fleaBay. The old one still works but just didn't seem to have any power to the motor.
Check to see whether there's any play in one of the spindles. I have a click at certain speeds and it's the bit that screws onto the spindle is worn so the click is when the play causes it to slip and rotate a bit.