02RSX could be right. If you have crappy tires, they could just have deteriorated from use. Basically, a tire hardens from repeated heatings, and if you have a Korean tire, that happens about a month after you buy them.

There's a million other reasons if it's not crappy tires..it's hard to say since we can't hear the sound. The simplest things would be:

-underinflated tires
-worn tires
-alignment
-wheel balance

Have you hit the curb or a big pothole recently?