Audio within a video clip makes grinding sounds every so often

Hi there

I have a Premiere project running through to just over 1.5 hours. It contains a selection of video clips, pictures, and music connected with various picture files at certain times. 

For the most part, the file is playing fine - however, there are parts where the audio sections of certain parts produce a type of grinding/buzzing sound so that one can only hear that buzzing sound instead of the actual audio that is present on the clip. If I play the clip in a stand-alone mp4 program, the file plays fine. There is no buzz - just the actual audio of the person speaking. When I import the file, these strange buzzing sounds appear - funnily enough - in the exact same positions even if I remove the file and re-import it again! The files play perfectly in a stand-alone - but once imported they produce this strange buzzing type of sound in place of the audio.

Can anyone advise me what to do or why this has happened? I have never had this before and I don't know how to get my file to play normally again.

Thanks.