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