In some parts of the United States, including the part I grew up in (the Northeast), they are pronounced _exactly_ the same. In other places, "aunt" is pronounced almost as spelled, with a diphthong.
You can verify this with an English dictionary. For example, the American Heritage dictionary, using its own set of symbols, says that "ant" is pronounced "ănt," and that "aunt" is pronounced either "ănt, änt."