If exiting to the right side tray is making the noise, the only difference there to a normal double-sided page is the diverting gate/solenoid and motor to pass the paper straight through instead of down into the ADU and side door. Run the motor tests in the ADU in sim 9. I'd say there is a problem in one of the motors in the side door. Maybe a problem in the motor bushings, which would explain it working only sometimes.
As for the fuser units $$, I have no problem with that. Always got a new one or a full refurb in the car ready to go.

Sure makes diagnosing weird noise issues easier.