Whilst using my iPhone 6 a few days ago on picking it up I noticed the screen makes a clicking sound near the bottom corners.
Would appear that the screen is loose somewhere or has a manufacturing defect. Looking at the official Apple forums there is quite a big thread that makes me think it could be more of a common problem than we think. it's an odd issue that I would put down to manufacturing fault but based on the volumes they sell it's not surprising that there are some problems.
I'm going to book a genius appointment in the coming weeks then I will update this post with the result.