What exceptions exist to the leading "sound" rule for use of "a" or "an"? An example would be: "Provide a unit in the metric system for measuring length." Here "unit" leads with a vowel sound (i.e. long "u") but the article "a" is used instead of "an".

It's funny because I was always taught there was no "rule" persay... But it is always easily identifiable when you speak the words. So what I was taught was to say the phrase in my head. (I was never taught a "rule" and frankly, I dont think one exists...) "an hour" is another commonly used exception.

I know this really doesnt seem like it helps, but it should cause thats all there is lol.