All one has to do is observe how a governor works on an ICE. As the load increases, i.e. going up an incline, carrying more load, encountering thicker/taller grass when mowing etc., the gov will allow the throttle to advance to add more fuel/air ratio inorder to maintain the same speed (mph, engine and pto rpm etc.). As the ICE burns more fuel/air all it's outputs (hp, torque and sounds) as well as the intake sounds will increase.
As been stated it all has to do with the ICE's load or lack thereof.....
Dave
