I put a 9,000 btu air condiotioner in the bedroom 6 years ago. We dont use it much, maybe 10 days a year.

The outside unit is starting to vibrate more than usual.

Other than cleaning the filters, is there any other maintainance that should done to an air conditioner?

If you want the unit to operate as efficient as it can based on you're usage of 10 days a year I would think annual service should be done on the compressor unit outside to make sure no critters are making a home in the box and that the condenser is clean and free of anything. The inside unit most likey is clean based on that usage again but it should still be checked for anything unusual and then a operational check to be sure all is working properly and it cools down the room as needed. Possibly the service guys can put their pressure gauges on the system to make sure it is OK but if it cools properly the freon and pressure should be ok also. no more than 400 to 500 baht for this annual service.