Hissing Noise in Dry Type Transformer

If you are unable to shut it down, I suggest the following:
1) Are you are sure it's hissing (air) and not buzzing (arcing)?
3) Have others given their opinion?
2) Try recording it with a microphone and then view the output on an oscilloscope to determine frequency?
3) Perhaps your facility or another facility nearby has a piping air-leak detector, you can use?
Regard,, Phil Corso
Interestingly, a hissing sound only comes from an air-leak ! Fluke, has an instrument called the "Ultrasonic Acoustic Detector" that allows a maintenance tech to literally "see" the leak" !
Now the interesting part... the sound made by an air-leak must be greater than 60 decibels to be heard by a human... meaning it must be a sizeable leak !
Regards, Phil Corso