i don't guess it's really a problem as much as a phenomenon...

Welcome g-cart and dood44,

As you might know, sound is caused by vibrations of air, like ripples in water (sound waves).  When you sling an oddly shaped rock, it will likely tumble because it is not aerodynamically stable.  As it begins to tumble, the odd shape basically "slaps" the air as it rotates, causing a vibration.  Because it tumbles over and over again, it makes a proper wave form, causing a humm.  You might notice that smaller, less dramatically shaped rocks produce higher pitched sounds.  This is because they rotate faster, and make higher frequency waves.  Larger or really weirdly shaped rocks tumble slower, making a lower pitch sound.  

Chris