surasshu said: Vowel sound, so it would be "an"! Unless you say howrglass.
This is correct. Words that begin with silent consonants (or a group thereof) defy the rule.
fcma172 said: If I remember my English lessons correctly, you only use an when the noun that follows it starts with a vowel.
Note that it's the next word, not the next noun. "I have an apple." is correct, but "I have an red apple." is not. The word "red" is an adjective, but it requires the use of "a" over "an".
You people are making this way more complicated than it is. Simply put, you can't have two vowel sounds right next to each other. It's difficult to pronounce.