Are you the original owner? Do you know if a previous owner may have used a stop leak product to fix the radiator or used a coolant that isn't compatible with BMW coolant? Could be the heater core is blocked. You can try using a radiator flush product to see if that helps or not. I would drain the radiator and save the coolant and refill it with fresh water and a radiator flush product. If you live in a part of the country where the temperature goes below freezing I will wait until it is spring or summer to do this.