Mine whistles too...at first I thought I had a leak but I cant find anything anywhere. The only thing I can think of is that it has a resonance point. If you think about it, with the trottle at a certain position it would look exactly like a whistle. Once I got used to it, I really like the sound of it.
sounds like you guys installed a helix style spacer. the whistle is normal. it is caused by the spiral design on the inside of the bore. so dont bother looking for leaks, youll just give your self a head ache.:thumbup: