I have just fixed a most annoying tailgate rattle on my 2014 CX5, and as it took me some time to locate the source of the problem I thought I would share it. The sound is like a box of tools rattling in the boot when going over small bumps or an uneven surface. I eliminated everything that could potentially been rattling in the boot and even under the back seats to no avail. After searching this forum and finding no threads about this I found a US CX5 post about the external plastic moulding above the rear window - they call it the 'spoiler' - working loose and causing this problem. For some reason the spoiler's 10mm fixing nuts, underneath two rubber grommets on the inside on either side of the tailgate (and also two others underneath the interior moulding) work loose. I found one of mine was missing completely. so replaced it and the rattle got better but didn't go away completely. I then noticed the two external triangular plastic mouldings to either side of the spoiler were also loose, but they appeared to be fixed to the spoiler and could only be investigated by removing the spoiler altogether. This wasn't possible as it looks like a special tool is required to release the spoiler's additional nylon snap fixings to the tailgate body, otherwise they will break when levered off. So, I simply wedged some foam rubber pads underneath the lower pointed end of each moulding between it and the glass, and now its solid and quiet! I figure if this assembly is loose in any way, it either rattles against the rear screen going over bumps or even flutters in the airflow, and it sounds just like something is loose in the floor area so is really difficult to locate.
