Feenix,
It's nowhere near the sound of an ac, about the same decibels as a noisy old desktop pc. With my two 150W units if I don't keep the built in robe's doors shut (my tent is inside that) then you can clearly hear a whine, with them shut, no problem. So, if you expect to have people in the same room as a stealth grow you may hear a faint sound, unless you can contain the cab in another structure, or keep the TV or radio on.