I have the same car, same year and had an identical problem.,
I would lay money that the "knock" you are hearing is the poxy piece-of-crud undertray wobbling about like a tin-roof in the wind. I swapped mine for an aluminium one (mostly because it was scraping over speed humps and the shabby look of it on a new car really annoyed me). And the sound was gone. Sounded exactly like it was in the front suspension.
Ask them to take the tray off and take it for a test drive. You'll know instantly if its the problem. If you have access to a set of jacks and mate to help you (it is a 2 person job!) then you could easily do it yourself.
On the ride, yes they are like that, the "cure" is a set of Koni FSD/Eibach Pro kit for ~£1000 including fitting, the stock suspension lets the car down IMO and can be made much better, but without turning it into a hard as nails track day car (quite the reverse). If you are interested do a search and you'll see dozens of articles on here about it. Don't get the dealer to fit it, find an independent who has done it before on a supply and fit basis. I can recommend the lads at Autolusso if they are anywhere near local to you.