- A stuck in the open air check valve (may not be the exact name of the gizmo)
- A ruptured air bag
- A ruptured hose
- A loose fitting from hose to airbag
- None of the above
Have someone do this for you in the cab, as you are positioned next to the tag. After starting the engine and let it run for awhile to build up air pressure. Try using the level reset button, then go into Leveling mode. If the leveling red light is blinking, then turn off the engine but leave the key in the 'on position'.
See if you hear the hissing continue, and if you can identify where. And does the electric compressor still kick on too?
And of course you know the CAUTION to not get under the rig, especially if you are having any sort of air and bag problem...
Those are my thoughts. Wish you well, and will follow as others add suggestions too. (Monday, you might call and see if you can get some help. They're usually pretty good at helping us mere mortals in debugging problems.)