Does this happen to all helicopters?
If it does, then shouldn't it be noticeable in autorotation as well? Of course the rotor will spin faster in the turn, but it should wind up faster when rolling right than left if there is some sort of resistance in the rotor-system, gyroscopic or mecanic. Or will it be too small difference to notice?
If there's no difference maybe that shows that the reason for the spike is on the engine-side of the freewheel and not the rotor-side?
Or probably I'm way off, it's known to happen
/2beers