Vito hissing behind the dashboard

[Brinyan]

I have a 2008 Vito Dualiner. For the past few weeks I’ve noticed a loud hissing noise when starting & after switching off the engine.
It’s coming from within the dash. A couple of days back, there was a short spurt of cloudy gas blowing from the forward facing air vents. I opened the windows, but it soon stopped.
Air conditioning still blowing cold.
Any ideas what’s going on?
Thanks