My cat is loud and obnoxious!?

I got two cats from the shelter April 2010. A three yr old black cat and a one yr old grey cat. They get along very well and my boyfriend and I give them a lot of attention but our grey cat won't stop meowing! She drives us nuts with her high pitched, obnoxious, lengthy meows. She's been spayed and I've taken her to the vet recently and there is no problem with her. She eats, drinks and uses the litter box regularly so I don't know what her problem is. We try ignoring her, giving the time out but nothing seems to work. Our black cat is not like this and I'm hoping that our grey is still young and will grow out of it?