I know what you mean by greater stress around people after being isolated for a while. It's important to "log some time" (if you're able) around others to maintain a better level of skill/comfort, just as it's important to have time away too.
Humming is not too uncommon of a stim. If it's causing issues for you, could you re-channel or repackage it? By repackage, I mean maybe try a tune instead of randomness. It may still not work for all settings, but people generally seem to ignore someone lightly humming a tune rather than random sounds.
Or re-channeling may be an option if it works for you to direct it from sound to physical movement. For example, no one seems to care much about someone bouncing their leg or twirling their hair (though more acceptable for girls, I guess).
Other options; if there's some small scented object you could bring to smell that makes you feel more calm, or even chewing gum to get the physical, taste and senses of smell.
Someone else may have better ideas, but maybe something here might help. 