Fresh outside air
After reading the many replies, none addressed the smell problem, BREATH.
My 1987 300E does the same thing if I repeatedly use the recirculate mode. In this mode your exhaled breath is condensed on the evaporator coils and dripped off into the cool/heat housing during compressor cycle OFF.That's when you smell you.
If you aren't doing this then your fresh air door is not opening.
Check it out.
Happy Trails Beep Beep from Houston!!!
Donald, El Cheapo