Underfloor noise - Thoughts please

Past weeks I have had a 'flapping' noise when 60mph + tried all trim inside etc, etc
Went under car today and there is a plastic 'cover' on the chassis almost under passenger seat, this appears to be cracked and I think this is the noise I can hear

I am only person who has driven car since I bought in new in March 2014- its NEVER been offroad and indeed never been off tarmac

My concern is that when I go to dealer I will be told 'my fault '

Welcome thought / comments please