I'm having issues getting my Arducopter Hexa flying stable, I here the props fluttering at the hover every few seconds followed by wobbying, and I observed possible vibration coming from the left forward motor. So on the ground at 1/4 throttle without control inputs I observe the prop nut vibrating every few seconds, which makes me suspicious. I've balanced all the props, and the motors are secure, I can't see anything wrong physically with that motor.
Its strange because its vibrating maybe every 3-4 seconds, its not constant.
Can you reverse two motors / esc and see if the problem remain ? (just invert ESC connections on the APM).
If you get the same problem on a different motor /esc, there are good chances that you have a jitter or signal corruption problem on your APM output (code problem or hardware problem). If not, you have a faulty motor or ESC.
Wich firmware version is it ?