I have a vibrator from a yucky sex toy store, and it's really loud. Every time I use it I think the plaster is going to drop from my ceiling or my roommate will come barging in wondering about all the ruckus. Isn't there anything quiet out there?
There sure is! Your best bet is the ultra quiet and equally reliable Tender Touch Massager. Plug it in, turn it on and you won't hear a thing -- you'll just see its tip busily vibrating back and forth. One caveat, though: After a few years of use, the attachments start to come loose and the toy can get noisier. If this happens, try wrapping a rubber band around the post and then sliding the attachments on. Problem solved.
Here's another trick: those standard, cylindrical-shaped vibes like the Classic Vibe are really inexpensive, making them a great choice for first-timers. But they can also be quite noisy. Believe it or not, the noise you hear is caused by the sound of the batteries rattling around in the inside of the vibrator. Open that puppy up and slide a folded piece of paper inside: you'll take up some of that rattling room and insulate the noise at the same time. You'll notice a distinct lowering of the decibel level.
You can try Candida Royalle's Natural Contours line of vibrators, as well. Candida made them expressly for women, and kept in mind that lots of us don't want a vibe that looks like a ding-dong and sounds like a jet airplane. Not only are her toys whisper-quiet, they come in pastel candy colors and discreet shapes. You can turn them on and your roommate will hear nary a peep.
The last option, of course, is to crank up your stereo, or convince your roommate that you are actually using a hand mixer in your bedroom. Or, buy her a sex toy of her own -- then you'll know she's having just as much fun as you are.