Jan,
I got lucky the chamber for the whistle was in my box of silver, this was originally made as an extra in case I had problems. I made these for Bolo tie tip ends, yes they are two hemispheres soldered together and I knew that it would work perfectly for the whistle project.

The hole was in the piece as if you don’t allow somewhere for the gasses to escape while soldering the two halves you might just have a small explosion on your hands. I was going to put a small cab on both sides in order to cover up the hole but when I got to the point of being able to listen to the whistle the hole causing pitch change made the whistle even a little more cool.

Below is an image of the Bolo so you can see the original purpose of the “bead”.

All my best .......... Danny

P.S. Not a chance I'm going into the whistle business, better beads than whistles. I am at the end of a major single bead project, once it is done I will for sure post it on BCN