CTS-V front wheel knock
 

I purchased a 09 CTS-V about a year ago with 8k miles on it. I have 25k on it now and when I am backing up and turn the wheel I get a loud knock from the front wheels. The dealer is telling me that there is nothing they can do about it. ??????? Anybody have a cure for this?
Thanks in advance for any help I can get.
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Ken