The whistle was still there even after my attempt to mitigate this problem.
The idea was that I could duplicate the path of the sound, L/R style, and have it cancel out where the paths combined. This did not work.
This may be my fault.
In the original mitigation design I had two equal length/volume paths for the sound. In other words I was borrowing from automotive ICE exhaust system design where they put an H shape between the two banks of cylinders to selectively add and cancel out particular frequencies of sound. Everything being equal (which it was in the original re-design), the waves from both sources should have been identical and canceled out. From that point within the ports, the internal path to the actual sensor went off at a 90 degree angle.
Then, I moved stuff around to generate a smoother transition to the sensor tube from both ports. This resulted in a now unequal path length to the sensor, and I believe that is why the whistle still exists.
Besides being annoying, I think this may actually be contributing to the noise I see in the data at high flow rates (when the whistle sound actually happens)
I have not decided what to do about it yet. I suppose the first change should probably be to go back to the equal length paths I had and see if that fixes it. If it does not, I could always move the whisling to below audible by increasing the 'dead space' for the cam position to sort it out, bor, but I really don't want to do that.
The whistle sound wave, is a pressure wave. It is definitely currently hitting my pressure sensors. My pressure sensors are capable of reporting at a good fraction of the apparent frequency of the whistle sound. This probably means whenever I do hear a whistiling sound that I am not getting the best data that I could be.
In any case, what we have works for testing... with the above potential caveats. In the interest of moving forward, I am currently willing to ignore this part.