I brought my 2016 in for noises in the rear of the car and they said there is a service bulletin about the rear stabilizing bar along with sound files to compare. Took the service rep for a drive and he heard them and they tightened bolts and put some sound deadening tape between parts in the rear of the car. Have not had a sound since.
sounds like something that might have been overlooked during manufacturing that isn't exactly the best thing to hear
fortunately for Mercedes the AMG GT is a well rounded product that makes up for its slight flaws so far.