An Innuendo

An innuendo has remained between them that is making their relationship more unpredictable.

Use "an" before a word that begins with a vowel <em>sound</em>.   Examples:  An apple.  An easy exam.  An honor.  An honest man. ["honor" and "honest" begin with the consonant h, but the h is silent.  Therefore, "honor" and "honest" begin with a vowel <em>sound</em> of the o.]    Compare:  a university     Although "university" begins with the vowel "u", it doesn't begin with a vowel <em>sound</em>, because the "u" in university is pronounced "yu".