My Read Aloud ePub producing popping sounds despite clean audio, any suggestions?

Hello Experts,

I'm having trouble with my Read Aloud ePub, when loaded to iPad and run through iBook, some of the pages produce pop sounds before the narration starts. I have separate audio for each page and they are clean. Audio don't have any popping or clipping sounds in it but when run in iPad it produce pop sounds.

Any suggestions?