Oh ..the dreaded whistler.. had a nice large but inexpensive full bent that did that .. I took a tiny ball end dremel and made the draw hole in the shank oval a bit by the "trap " and it never did it again i believe it was the edge of the stem making it whistle
Dumb question ..what does one clean a churchwarden stem with when its longer than pipe cleaners are and cleaning from both sides is insufficient in length ?