Aloud noise was heard (a, an) hour ago.*

The word that would complete the sentence, "A loud noise was heard hour ago." is an. The use of "a" is for the consonant and "an" if the first letter of the word following it is even. However, the word "hour", though it starts with consonant letter , h, it is still pronounced as though it has even first letter.

A loud noise was heart an hour ago. The correct answer is the article AN. Although usually the article A is used before words beginning with a consonant, and AN before words beginning with a vowel, there are some exceptions. Here, the word hour is indeed written with a consonant as the first letter, however, it is pronounced as /ˈaʊə/, which begins as a vowel when you read it, which is why it takes the article AN.