One of my breaker switches buzzes when I flip it on, should I replace it?
Buzzing breakers are not normal, but a common problem. The usual cause is an unseated magnet vibrating in the breaker. These magnets sometimes do not reseat into the correct position after tripping and resetting. This can cause the breaker to not lock into "on" position or hum under load. I am not sure if this trick will work on a AFCI breaker, but on regular breakers, we remove them and give them a firm slam on it's side. This sometimes jolts the magnet back into the correct position. You can then operate the breaker on and off a few times to confirm it is seated and operates freely.  I have not experienced this sound with a AFCI yet. So the sound could be the magnet vibrating or a defective coil. When I attempted to reseat a magnet and it didn't correct after a couple of "slams", or the condition returned after a trip again, I replaced the breaker.  Obviously there could be another problem and the breaker should be replaced, but the slam trick can be very effective.  

The other test is to move the circuit feed to another breaker temporally to see if the hum returns on another breaker (not likely), thus confirming the problem is in the breaker.