Mk8 Fiesta ST rattle - help!

Hi everyone,

I have a mk8 Fiesta ST that has a really annoying rattle that sounds as though it's coming from the rear of the inside of the car.

I've searched everywhere to try and find out where it's coming from but can't find it to be able to fix it.

It starts up when the car is up to temperature and then does it over smooth and rough roads. 

I've had it looked at by a Ford dealership who, as predicted, couldn't find it, but still charged £200 for looking into it. They wrapped the metal bits where the seats clip into in tape and did the same with the boot catch which has made no difference.

I've had the back seats down, the parcel shelf out, the boot floor out. I've even wrapped the B&O subwoofer in an old duvet cover to try and cushion it in case that was the cause but that hasn't helped. I've also checked the boot stops but they all seem fine. I've also driven with all the seatbelts plugged in.

The only thing I can't do is lift up the rear bench seat as I feel like I'm going to break something.

If I was to try and describe the rattle, it sounds like plastic rattling against something, making me think it's trim related, so I'm not sure why it only starts rattling when the car is up to temperature.

I know this is a long shot but has anybody else had anything similar and could you suggest anything?

Thanks for reading.