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Hi my name is Robert...
Few things I would look at.... First go topside and remove the shroud for the AC, be careful as well sometimes bees and wasps like to make nests under the cover. You will see the blades for the fan motor, make sure the fan spins freely. IF it starts at start up then I suspect the fan motor is the problem and would need to be replaced. One other thing I would check is to make sure the mounting bolts for the AC did not loosen up, if they did the AC unit would be very noisy. Pull down the inside shroud and you will see 4 or 5 very long bolts running up into the AC units. If they are loose snug them up and then put about 4 turns on each bolt, you do not want to crank down on them. I suspect on your issue if it is only doing it at start up the fan motor might be going out but will have to get up on the roof and inspect make sure the fan blade is not hitting anything... Hope this helps some... Robert
Sounds like the fan motor if it is happening when it turns on and shuts off, but I would verify the mounting bolts are snugged up. There is a torque on the bolts and it is only about 12" lbs, not ft lbs. Many times the bolts can loosen up to the point you can spin them back in with your fingers, so I would check that and then I suspect the fan motor is at fault, make sire nothing is coming into contact with the fan blades.... Robert
If it is squealing then it most likely is the bearing in the fan motor, what you can try is get some silicone spray and spray down the shaft for the fan and see if that helps quite things down at all, the motor should be able to be replaced if needed..... Hope this helps... Robert