The pilot reported that the helicopter was being hovered above trees when he felt a kick accompanied by a loss of power. He began to manoeuvre towards a clear area but had to pull up to avoid tree tops. The engine surged again and the rotor RPM began to decay. By the time the helicopter was over the clearing the "ROTOR LOW RPM" warning horn was sounding. Shortly afterwards, the helicopter struck the ground heavily in a level attitude. No fault was found with the helicopter which might have explained the reported power loss. However, the aircraft had been transported a considerable distance over rough terrain before it was examined. This could have allowed possible causal evidence to be disturbed.