How many miles? ,Its hard to believe that carbon would be an issue in a years time on a 2015. Sounds fishy to me,did they show you the what all this carbon looked like on the head? What exactly did they do the first three times to your car? Where is the knock coming from?I have a 2015 sonata that is NOT have a turbo. I've had an engine knock since about day one. Last month the dealer tried to correct this for the 4th time. They took the head off and sent it to a machine shop to be cleaned as well as cleaning the valves while the head was offf. They said they took more carbon out of my engine than they've ever seen. I haven't missed anything on my maintenance schedule and it's all been done at the dealer. They had my car for about 3 weeks and put 30 man hours into it. I've had my car back for not even a month and the knock is back already.