I purchased a Mercedes-Benz B150 automatic new just over four years ago. I currently have 91,000 miles driven. It is serviced at six-month intervals by a Mercedes-Benz dealership. Following a recent service, on taking car back I noticed a strange metallic (slight knocking) noise, seemingly from the passenger underside. At the dealership, the diagnostic technician's opinion was the gearbox needed to be replaced. I am told that to replace the gearbox (including labour) will cost circa £4000. Asking for possible causes, nobody within Mercedes-Benz can give me any plausible reason for the failure. My position is to part with circa £4000 without a glimmer of observation as to the root cause of the problem. This I find most odd. Any thoughts John?