Haven't had any direct experience with an A7 on 21s but personally an S Line with 20s should be fairly firm anyway. Plus, don't forget to check out the price of tyres... in the region of £250 each probably
I suspect not, a quick Google reveals only the common iPhones etc:
http://www.stokeaudi-parts.co.uk/categories/Audi-In-Car-Entertainment/Phone-Cradles/
Going to have to chat to your dealer I suspect...
If it was a warped disc you would get vibration through the pedal when braking from motorway speeds.
Knocking more likely to be a worn bush in my opinion.