Try lowering the setting on the thermostat to see if the compressor stops humming when it kicks off. If so, it may be going bad. It is also possible the rubber shock absorbers on the hold down bolts are worn. This may cause a humming vibration when the compressor is running.
If the humming doesn't stop when the compressor shuts down, then the fan motor or blades may be the culprit! Remove the filter inside the ceiling assembly. While the unit is switched off, reach up and spin the fan by hand.
The fan should turn easily with no abrupt stops. If it is hard to turn, the bearings either need oiling or a new fan motor is called for. If the fans are hitting some obstruction, you will be able to hear this while spinning the fan. I hope these suggestions are helpful!