Crackling noise on video iPhone 6 plus

well....Friday night i went to go see parkway drive who are a metal band, very loud, but the next morning I've gone to play the videos of them that I recorded with my 6 plus and some are making a cracklimg noise throughout but the others are fine. I have changed my phone today as it was under warranty and still does not play correctly. My mate was next to me and he has a iPhone 6 and his is fine. So it can't be a phone issue can it?

thanks