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Gas does make a hissing noise if forced through a small opening (think a gas leak or a can of compressed air). A Knockout Gas being sprayed out of tank making such a noise would be "realistic" and not really a trope. Gas that's just kinda floating there while making such a noise so the audience knows it's gas probably would be covered by The Coconut Effect.
Well, since gas in Real Life does do that (as long as it's leaking out fast enough through a hole that's small enough), I'd say this is definitely due to Rule of Perception, whether it exagerrates the effect to tell the audience there is a gas or it's just being realistic.
But it might be argued that these two tropes are not divided clearly enough.

I tried looking at Rule of Perception and The Coconut Effect but I can't find it.
Whenever there is a gas other than normal breathable air being introduced to the environment, there is a hissing sound. It can be a visible cloud of gas accompanied by hissing, or it can be only the sound to let the audience know something is happening. Often used for Knockout Gas being introduced to the environment. I have also seen this when air is being introduced to an environment without breathable air, as in Deep Rock Galactic.
I have seen this is television, animation, movies, and video games.