AMG-ED

MY 2018 GLC63 sometimes makes a rough grinding noise going into reverse. You can actually feel it. Feels like it's directly under the front seats. Mainly when the car has sat for a while and is cold. It doesn't do it all the time just sometimes. It doesn't do it shifting into reverse but actually when the car starts to move in reverse. Do you know how brakes sometimes feel releasing when they're wet? Feels like that.... but under your seat. I've never had a 4 Matic+ before but I wouldn't think this is normal. Any of you ever experience this?