Almost all babies outgrow the habit. Until yours does, you can give her plenty to do and look at, and offer her a chilled teething ring. If she's still grinding after all her teeth come in, have a pediatric dentist check them for signs of excessive wear.
Your baby suddenly sounds like a pint-size sawmill: Don't panic! About a third of kids grind their teeth at some point (it's called bruxism) -- especially when their front choppers start to cut through around 8 to 12 months. Rubbing those sharp edges together helps relieve painful gums. It could also be a learning process: "Having teeth is a new sensation," says Jeffrey Janoff, a pediatric dentist in Venice, FL. Grinding's one way a baby can explore what they feel and sound like.