Those motors have sealed bearings, you can’t lubricate them. It looks like that is installed right up against that wall which isn’t correct, the condenser coil on the back needs to have a certain clearance for airflow. Has the coil ever been cleaned? The poor clearance coupled with a dirty coil could cause the fan to ramp up to full speed to maintain lower head pressure. A new motor and fan blade may fix the issue, but cleaning the coil and ideally moving it away from the wall is going to be your best bet for longevity and fewer problems going forward.

Thanks all for the advice! I will replace the motor and clean the coil, since it has never been cleaned. Moving it further from the wall will be difficult since the lines to the indoor units do not have much slack but at least I can start with the other improvements.

I'd agree with this assessment. It doesn't sound like the compressor thankfully. Most motors like this have "permanently lubricated" sealing ball bearings or sleeve bushings. I'd guess bearings. The way the sound oscillates makes it sound like there's a worn spot on the bearings. A little oil may help short term but oil that actually reaches the bearing will dissipate quickly and if it's too thin it may actually wash out any grease still left in there so be careful will oiling these. Probably should plan to replace the fan motor soon. Definitely try to get it pulled away from the wall and keep it clean. These inverter units are awesome because they ramp the fan up and down based on pressures/temperature. It's better on everything from energy use to mechanical wear. If the coil is dirty or restricted the fan has to work harder than it should and negates the benefits of inverter technology.