Yes definitely the gun ports - the Mk58 has different gun ports and does not make any blue note at all.
Ref the Canberra PR9,I know the sound you mean and I guess than any aperture might make a similar note at certain airspeeds/relative airflow angle.
I will try and find a better meteor sound...Puddy Catt used to do a good blue note run at Brawdy with the F8 when returning from target towing etc
