Confusing, isn't it?

When you touch him, & he makes this sound, that's probably just his stress response. Not exactly a hostile "hiss", but just "sucking in air" as you described. I wouldn't worry about that as long as you're not seeing bubbles around his mouth, or him open-mouth breathing, & as long as you're not hearing this all the time, when not provoked.
An RI can cause wheezing sounds, but also crackling or whistling sounds with each (or with most) breaths, & that doesn't sound like it's the case here.
Another possibility, besides hissing or an RI: he may also have some old shed skin stuck in his outer nostrils from he last shed cycle. Make sure you raise his humidity to about 75% during his next shed- the nostrils are easily plugged up when the humidity is too low, as those flimsy little bits of skin are easily left behind- and then, they make some noise when the snake takes a deeper breath. So that could also be what you're hearing, & snakes don't know how to "blow their nose" to fix it. Sometimes you'll even see a tiny tag of skin in their nostril- & if you're really careful, you might be able to catch & remove it with small tweezers, but humidify more first, so it lets go.
Right now, I'd say this is just a "wait & see"- & probably not an RI. Good question &

for asking.