FCP is adding pops and clicks to audio tracks

I have some .mp4 files recorded on Full-HD Video cameras at 1080x1920 25p

When I play these files in QuickTime they play perfectly.
If I use QuickTime to trim the files and save the results - again they play perfectly.

If I import these into a 1080x1920,25p project in FCPX and trim them, then 'share' to Apple 1080 - the audio has pops and clicks.