It took me forever to figure this out.

If you want to know if your housemates can hear your vibrator, don't pay attention to how loud it is when it's two feet away from your head. Turn it on max, set it down, walk out of the room, close the door, and then see if you can hear it. It's surprising how much difference this makes.

However if you ever make noises in response to your vibrator you're doomed and can only masturbate when everyone else is out of the house, because "probably just some distant highway noise" never goes "oooohhh."