This sounds like you may have a valve spring that has broken.
What happens is the valve does not get noisy in most cases because only a small portion of the valve spring is broken off and it is still applying pressure to the valve when the cam passes its open duration period. As the motor fires a small portion of the flame (explosion) escapes the valve past it and the seat.
Eventually this will cause a point of the seat and valve surface to wear and the motor will get to the point where it will not operate.
The reason it seems to stop doing this under a load is the pressure exerted by the load tends to increase the pressure of the explosion to a point where the explosion itself applies enough pressure to the valves face that it seals better and nothing escapes the combustion chamber.
The other thing is that you may not hear it due to the noise of the blades when they are engaged. I doubt that this is the case though.
Do a compression test on the cylinders. This will show if this is the problem. Ant discrepency in the height of the valve adjusters will be a definate sign of a busted spring.
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