It sounds like you have a balance problem on your blower motor. You didn't specify what type of A/C you have, but most rotary cage blowers are similar. You should unplug the trailer or switch off the A/C breaker, get on your roof, remove that A/C shroud and look in the central section of the unit where the blower motor connects to the frame. There are 4 bolts that attach the mounting plates to the frame, and 4 very long horizontal bolts that attach the motor to the plates. Tighten ALL. Then rotate the blower cage with your hand while holding the shaft to see if there is any play in the bolt that locks the cage to the motor shaft. Tighten as necessary.
That should solve your problem. If not, we have to dig a little deeper