When you service the AC's and have the shroud off the top of the AC take the time to turn the motor shaft by hand and see how hard it turns. Also check to make sure that squirrel cage exterior fan blade is not hitting anything or has any mud dabber nests in it causing it to be unbalanced.
Last year my front AC unit was kicking out and was louder then the back. I checked and fount that the motor was very hard to turn. I ordered a new motor and when I was changing I found that the fan was hitting the shroud, the motor mounts had failed and let the motor sit lower then it was suppose to. Put the new motor on and everything is working great & quite.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi