Vent. It's a slit in a garment, usually in the lower edge of the back of a coat, to allow easier movement.
Or maybe it's an opening through which gases, especially air, can pass. Or the opening of a volcano from which lava flows. In other words, a conduit and/or orifice through which volcanic materials (lava, gas, and water vapor) reach the earth's surface, with that distinctive, characteristic sound.
Here are a couple further thoughts to shove you along...
(1) If the vent in your jacket is emitting lava, then you've overheated.
(2) I let it out at the vent, or it let me out, depending on the point of view. Anyway, it fits better now and I don't feel bloated any more.
Take your pick, based on how you feel today, and what you've eaten lately.
Have anything worth adding? Then try sosayseff+eff@gmail.com
Me? Recently got blown off. (The usual.)
Etc...
so says eff: sporadic spurts of grade eff distraction
definitions: outdoor terms
fiyh: dave's little guide to ultralight backpacking stoves
boyb: dave's little guide to backpacks
snorpy bits: nibbling away at your sanity
last seen receding: missives from a certain mobile homer
noseyjoe: purposefully poking my proboscis into technicals