can anyone tell me why gravel sometimes hums when you sling it?  has anyone else experienced this problem?

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

Its the air passing by it when its spinning fast, or just going fast really. Like when you swing a stick through the air.

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

Ha yes, thats what I said. And thanks for the welcome Im just starting at slinging. Im about to make a braided one since my ghetto sling made out of string and duct tape just isnt cutting it, although it works.