Funny you should say that, because on his first flights he had the end of the turbine exhaust 1.25" forward of the opening of the BELL of the tailpipe instead of the from where the actual tailpipe started. We were sure that was it. But the next day he moved the turbine back some 2 inches to be only about 3/4-1" from the start of the tailpipe, and the whistle was exactly the same in flight.
I'm pretty sure it is either an airframe whistle, or the servo arms he is using. They are Futaba, I believe, and the rudder one might be at just the right angle to cause a whistle with air passing through it:
https://www.google.com/imgres?imgurl...MygsegUIARCyAg