A vowel is used when the word for which it is used is the sound of ‘a’, ’e’, ’i’, ’o’, or ‘u’, and these words are pronounced with your mouth open. These words are accompanied by the article ‘an’.  If the word has any other sound to it, apart from the vowels then it is called a consonant and the word is to be accompanied by the article ‘a’.  Going by the explanation, the word aunt has the sound of ‘a’ in it and therefore it is ‘An aunt’ and not ‘a aunt.’