Buy an air filter and place it in the room. They generate noise. This will cover the sound of your exhaust fans.
I haven't done this with roommates, but I think its possible.
Buy a key lock for your bedroom door (nothing out of the ordinary many people do this with roommates).
If your using an hps youll need a decent fan to get the hot air out of there...so you will need something with a decent CFM, but yet quiet.
S&P inline fans would be the way to go.
then again like everyone said what are your roommates like? will they freak and go call the cops? steal your plants? tell more people? want to smoke?
if you put a key lock on the door, and keep them out i duobt they will figure it out...like someone said get a fish tank, or an air filter like i suggested and put it in your room by the door...air filter will not only make noise which is good but will also help grab any scent that makes it that far =]
you will def need a carbon filter!