My seven year old Mercedes E350cdi has developed an odd problem where there is a smell of diesel fuel in the car when the engine is running and the car is stopped. Truth be told it smells more of exhaust/diesel fumes. Now when the car is moving the smell vanishes but when stopped at the lights it builds up and can be quite bad. Not the extent where it makes you feel unwell, but enough to make you take notice and think, god that smells bad. I cannot see any evidence of a diesel fuel leak inside the engine bay or around the fuel filter, nor can I find evidence of injector leaks. What could it be?