Carbon build-up can indeed cause the symptoms you describe. I've had dozens of cars in this winter with the same cold-start/noise issue you describe. I normally use GM Top Engine Cleaner to resolve these complaints. I've got a 2002 E55 in the shop right now with the same problem, although this vehicle is constantly used for short trips. We inspected the pistons with a borescope and found heavy carbon deposits. We decided to try Techron so we added some to the fuel tank at about three times the recommended amount and drove the car around for a few days. It made an immediate difference. Inspecting the pistons again showed the carbon deposits were nearly gone. I can't explain the reason why your ML has an on-going problem, but at least the fix is simple and cheap. If you decide to use the GM product make sure you change the oil and filter afterward. Good luck.