Rattling/vibrations from under steering wheel in footwell - found where but how to fix?

Hi All,

Had an annoying rattle/vibration develop going on for a month now in the driver footwell area, I assumed was plastic panels but couldn't find anything obvious. Doesn't seem related to revs, but as soon as the car is going about say 20mph, and still persists if I go into neutral. Managed to find it yesterday by luck, there's an area before the footwell air vents, if you come further towards the driver seat side, below the steering wheel area, that seems to have a semi flexible thin-ish sheet of plastic over it. If I put my hand on that sheet and keep it pressed up whilst driving, the rattle/vibration stops.

No idea whats under here or how easy it is to do anything about, at a guess I'm wondering if there's a loose cable in there? Any thoughts/suggestions would be a help before I start trying to open things up!

Thanks!