Hondero did you work on the subject lately? I've made some experimental ones,still searching,progressing slowly,but not giving up,here are 3 designs,that i'll test today.The one on the left is a hollow lead projectile with two 4.5mm holes on it's nose.The  middle one is a standard lead gland of mine with an aluminum sheet "bullroarer" hafted in it's tail.The one on the left is an 8g Co2 canister with a lead jacket on the outside,added for weight,and two 5mm holes on it's nose.The one on the left has a nice and loud whistle if i blow it,this design is the trickiest to make but if i find the hole size-cavity volume ratio it must work.

a golf ball with a hole through the middle whistles quite loudly, i would say about 7mm dia. pretty much anything that interrupts the air flow will make a noise. CO2 canisters work well tho, do u use flights to make them fly in a line?

Jlasud, nice to see you continue working in the project, your last designs looks very interesting. I have not deserted but recognize it is difficult and have taken a rest after making many designs and tests without getting the projectiles really to whistle loud. I have tested a lot of clay hollow projectiles, with a good hole at the point, of 1 cm approx, so that making them fly with some inclination they sounded like when you blown in a tube. They sounded very well when blowing, but when flying they only made a weak whistle. The problem is that at flight the air does not enter very much concentrate in the hole so as when it is blown. I´ve even tested a hollow projectile open at both ends, with a plastic whistle adapted in the interior, ha ha, and the problem is the same, an air flow very little concentrate. The advantage of an arrow is that it get always a perfect orientation at high speed, which is almost impossible to get with a sling projectile.  It is necessary to continue working, perhaps in others directions,  and if we were more people doing tests and designs it would be easier. Many people have theoretical good ideas but they do not make the projectiles and nor test them. I encourage them to do it. It is funny and would be fantastic that in slinging.org we were able to design a true whistling projectile. Perhaps it would be necessary to institute a prize to the innovation here, although it is an honorary prize,  

ok, imagine a clay glande, and on the exact  middle, ringing all the way around are  22 shells, thier open ends just barely exposed

22 shells whistle loudly when slung. i will make one and try it. my imagining is that it wuold scream loudly as long as it is traveling fast enough.

How well does the film canister arrow whistle?

I like the sound they make as they fly. It has a nice ring to it.