Rear doors lower sill covers rattling

I have noticed a faint rattle coming from the rear door area on bumpy roads and upon investigation I noticed that tapping both the rear door sill covers that they rattle. The ones on the front doors are solid and sound firmly attached when tapping. It would appear that the bottom edge of the sill covers is where they are not very firmly attached but I cannot see how they are attached. They are not falling off - just loose enough to rattle.

Anybody else noticed this? The car is due for a service in a couple of months time so could get the dealer to look at it, but I could also try and get some silicone in the gap on the bottom edge of the door. Suggestions please.