Mine did the same. Contacted MB dealer front brake pads replaced under warranty (the car had only done about 2000 miles). There is a technical bulleltin from MB quoting it's a design fault. The replacement pads solved the problem.
I also have the problem of "crabbing" front wheels. MB have agreed to change all 4 tyres under warranty. I'm not convinced this will cure the problem!
Auto stop start not working correctly (been to the dealer twice) waiting to go back to dealer again.
Artico leather (Vinyl) worn (only 6000 miles on the clock). MB refuses to deal with this.
For a £40000 car from a prestige manufacturer I'm not impressed.