Mine front AC was doing the same thing last year. It was louder and had a harmonic type sound. It also started to kick the breaker upon startup.
After we got back from a trip in Sept I decided to take a look. I took the shroud off and could barely turn the motor by hand. I also found that the squirrel cage fan blade was actually hitting the sheet metal.
When I pulled the fan motor off/out I found that the small rubber bushings that support the fan motor had worn to the point that it let the motor sit lower causing it to hit the metal. You don't have to completely take out the entire assembly, I was able to unscrew the set screw that held the front blower unit on and just slipped the motor off of it. This saved me tearing apart the house side of the AC unit. I also found that the motor bearings could not be replaced, they were sealed into the end casing of the motor assembly.
I went to this site to find the correct part number
Enter the model # of the AC unit and it will bring up a parts list/diagram. You can then find the part you are looking for. NO guessing.
I then did a search on the internet for a replacement. I found an open box special at Adventure RV for ~$110 including shipping.
In all it took me about 4 hours to fix but considering what a new unit cost it's worth if for a DIY. Besides, I've read that newer units are failing quickly, there has been some sort of production problem.