Audi dealership bad service

I had a terrible experience with the Audi dealership at the Glen. I purchased a brand new vehicle from the dealership, however after 4000 km, a knocking sound developed. I took it to the workshop ,however they haven't fixed it.. They have have received my car on 4 occasions in the last 4 months, however they cannot isolate the issue and refuse to resolve it. They have recognized the sound and admitted that there is a problem, however nothing has been done. I am disappointed as I thought buying a new vehicle would mean I wouldn't have any car problems, however the opposite has occurred.