You will have to get on your roof and remove the A/C shroud. If it's a new unit, it's probably a Dometic DuoTherm. With the A/C breaker OFF, check the compressor hold down lugs and spin the squirrel cage at the condenser (it faces the rear) to see if you get the noise or any other feel of an obstruction. Also go over the sheet metal screws to see if all of the sheet metal housings are making solid connections. Also look for pest infestation anywhere you have access and check the 4 screws that hold down the fan mounting brackets to the floor pan and then the 4 bolts that go thru the rubber mounts in the brackets. If any one of them is loose, it may cause a vibration or random noise like you have. Finally, remove the air distribution box on the interior ceiling of the T/T (where the filter is) and locate the 4 mounting bolts that hold the A/C to the roof and give them a crank with a socket wrench....just in case they came loose or were improperly set at the factory.
Action, FYI, I have never seen motor bearings available for newer A/C units and it has been my experience that one does not replace fan bearings....rather, the entire fan motor gets replaced per factory policy.
Lew Farber...RVIA Certified Master Tech...ABYC Certified Master Marine Electrician
SOLAR TECH ENERGY SYSTEMS, INC.
AM SOLAR ...MAGNUM…VICTRON ENERGY...LITHIUM & LIFELINE BATTERIES…MARINE REFRIGERATION