Engine Noise problems of the 2014 Mercedes Benz ML350

I bought this vehicle new on March 2013. With the time we hear a strange noise, we thought it was normal but that noise was stronger so we went to the dealer garage isla verde Mercedes Benz, to the maintenance we told to the person about that noise, so they said to us that everything is ok then we ask about the noise in the engine and they told that "is a problem in the engine and Mercedes Benz knows that, is an intern engine problem and there is a part that can't be changed! and also others models like glk, e and ml have the same problem". . So I'm worry about this because the warranty is only for 40,000 miles. . So what I can do????.