okay someone out there should be able to do this.
 
Lots of cultures have produced whistling and trilling arrows, so why not a whistling sling bullet ?  
Got the idea when an irregular shaped stone I slung last night whizzed away making a trilling noise.  
 
Bear in mind that gland shaped missiles spin, so  the holes could be either on the front on on the sides (bit like scoops - quick thought, drill hole partway through gland add side scoops and you've got a primitive jet engine :-)
 
Anyway that's the project folks - make a singing missile :-)  
Oh yeah and paint it flourescent yellow so you can use it more than once :-)

I've been slinging nails, works really good if they are straight.
They make whistling sounds, even if they don't spin.
 
Gard

If you don’t want to sling nails or stones with holes, an excellent choice would be nuts. Not the kind of nuts that you eat, but the kind of nuts that you find on mechanical things, like a tractor, car or whatever. Don’t ruin your cars of that reason.

At which point they would be passed off by law enforcement officials as "raving lunatics" and they'd never know of our dastardly plan!  
MUAHAHAHAHA!!

if you sling a flat wide rock with side arm it will make a cool sound like....uh..  
fugggkaaaaaaazzzzzzz......boom

Wait, so what do I do? Do I put three holes or four and where? Set up like a plus sign, a "T" or equally split? I wanna make some sand glands like this...