I noticed a very faint buzzing/rattling noise when I first got the car two weeks ago - only really audible if the radio was off or quiet and the road wasn't very smooth.

Tracked it down to the passenger seat headrest. On non-smooth roads, the seat back was vibrating when no-one was sitting in it and it was actually the headrest that was making the noise. You could replicate it at a standstill by giving the seat back a good "tap" to make it vibrate.

Had the idea of swapping the front headrests (I know they're asymmetric but it's only the button so doesn't really matter) and it now rattles less and not all the time but it's still there occasionally.

Not a huge problem but does annoy now and then and wondered if anyone else had noticed it?