Cranked up my AC units after being in storage over the summer, and the rear unit sounded just awful - like a bearing going bad.
While I was washing the roof, I hosed down the units and saw a bunch of sand running out of the rear AC unit, and knew what the problem was.
Mud daubers in the the fan rotor!
Also discovered a bunch of dead insects, spiders, and really clogged coils which got rinsed from the inside out. So I've added yet one more task to the annual maintenance list...
So if your AC ever sounds really bad, now you know!