That's a great question! One might think that your cat is vibrating because her ringer is turned off. But that's not the case.
Your cat vibrates for many reasons - I'll list them and give you the remedies - you should try them all just to be safe.
1) She's dirty - Cats vibrate to try to get the dirt off themselves. You should give her a bath. Cats LOVE baths! Be sure to remove your clothes as well and lock the shower door before turning on the water - this will enhance your enjoyment of the cat-bathing experience.
2) She needs a new medicine. Go to the vet and get some cat pills. It doesn't matter what kind, 'cuz the important part is that you give your cat a cat pill. Cat's LOVE to take pills. This should be done in the nude as well.
3) She wants her belly rubbed. Cat's love an aggressive belly rub. They love it so much, they will hug your hand while you do it. This too, is best done in the nude.
I've made these helpful tips to help you keep your cat from vibrating too much. Do these, and I guarantee, she'll stop vibrating.
For The Humor Impaired: For the love of GOD don't do these things, ESPECIALLY in the NUDE! Cat's HATE these things - the lacerations you will receive will likely be bone-deep and require extensive surgery to repair.
If you do not know why your cat is "Vibrating" - then you probably shouldn't own a cat. Find a single lady in her 50's - she'll likely understand what your cat needs and will take much better care of it than you will.