Converter or hot muffler-like smell entering cabin


This is an 2001 BMW 740iL. 65K miles. Its a CPO still under warranty and has been a great car except for the odor that seems to enter the cabin through the AC system vents. The car did have a leaking radiator which was replaced. That was a source of one odor, but there is another that literally gives me a headache. I smells a bit metallic and like fumes from a hot exhaust at times yet it is also has a unique component that smells like a hot fluid such as power steering or anti-freeze. The dealer pressure checked the cooling system and tells me there is no heater core leak. I have replaced the fresh air filters under the hood. I thought there was a gap and leak around the filters, but they appear to fit well. The smell occurs only through the drivers side vents. It becomes more pronounced if the air is on low and then I just crack a window or open the sun roof just a little. For a while I thought that exhaust was somehow drafting back into the car. So far the dealer has been useless and expensive. Do you have some suggestions on what this might be or how to track it down?