Knocking i30

My Hyundai i30 has a knocking noise in the engine bay that neither the mechanics nor a team from Hyundai could fix in the three and half weeks they had the car. Hyundai's response was that they can't fix it and they don't know what's causing it, but they've confirmed that all and only petrol/manual i30s have this fault. In my opinion new vehicles with faults that can't be fixed shouldn?t be allowed on the road and the carmakers should be threatened with prosecution if they do let them out on the road. Some people will buy new vehicles based on the warranty, especially if it's five years, but I feel I?ve been conned by a carmaker that isn?t honouring its warranty. I'm stuck with a faulty vehicle that "supposedly" can't be fixed.