Low-frequency wobble

The low-frequency wobble is an oscillating rolling motion (that is, the heli wobbles about its length axis). In my case the amplitude was quite small, maybe ±5 degrees, and the frequency about 4 Hz. When this occured with my Raptor 50, I found that the head speed was quite low, about 1500RPM or 25Hz. This problem may be caused by unequal tightness of the main rotor blades. Alternatively, the problem can solved by increasing the head speed. A friend of me has the same problem with his Robbe Futura 60, when he hits 1430RPM, so this problem is not unique for the Raptor.
The first explanation I thought of is the phase difference between engine and main rotor. For the Raptor 30 this could be an explanation: the rotor to engine ratio is 86:9. 86 and 9 have no common divisors, therefore the engine will return in sync with the head every 9 revolutions of the head. As the head rotates 25 times per second, this could result in a 25/9= 2.78Hz problem. Slightly under the frequency I noticed, but maybe I estimated it wrong.
However, the engine to main rotor ratio of the Raptor 50 is 85:10 or 17:2. Therefore the engine runs in sync with the main blades already after two rotations of the main blades. As the main blades rotate 25 times per second, I would expect a 12.5Hz problem and not 4 Hz.