Hey, that sounds like a whisper key pad. (You can turn the bocal away from the pad to check that or you could also try another bocal). When the pad gets old and dry it can do that, sometimes even a new pad does it. The other cause can simply be the pad is too close. I have a student with a Walter that has the whisper pad too close, it hisses. You could also try removing the key to see if the hissing stops. A new pad might do the trick, years ago I didn't have a spare pad and I glued a small suede disc to the pad and that did it. I put a Valentino synthetic cork pad on my daughters whisper for that problem, also because she kept tearing the leather off the pad.
Principal bassoonist, Orquesta Sinfonica de Galicia, A Coruña, Spain. Bassoonist, bassoon dad, bassoon husband, bassoon uncle, bassoon brother and bassoon son.