I noticed while out flying today while I was hovering the aircraft in front of me the whole aircraft was shaking pretty hard, like a heavy vibration. After a while it would slow down and come back periodically. Whole aircraft is new with only just a few flights and never a collision. Props in perfect condition. What could cause this? IMU? Bad motor? Or will a out of balance prop cause that much vibration? Again, it comes and goes usually within 20 seconds.