The car in question is a 2011 3 hatch six speed. It has a little under 20K on it. I've started to notice that whenever I use the defroster it gives off a bad smell. It is a musty kinda smell. I only get this smell with I'm using the defroster, whether it's the windshield only option or the windshield and foot option. Any ideas as to the case of this? Could it be my cabin air filter? Thanks in advance.