Air

The a/c evaporator may have dirt or mold build up. Talk to your dealership to see if it can be cleaned.

Replace the cabin filter too

Yes, do just as the previous answer say. I would try the cabin filter first.

also try to put it in the fresh air mode..

Cabin filter. Tom can't be correct as there's absolutely no history of heater core failure in these Subies.

I have to respectfully disagree. There is always the first time for everything.

The reason that the heater core never fails is that the head gaskets usually blow out and coolant never reaches the heater core on these cars. ROFLMAO!

I recall another member on here talking about their allergic reaction to some of the composite materials used to make the interior panels and such. You might try looking at that along with the cabin filter. Hope you find the cause.

Possible exhaust leak.

Yeah...probably from Bob; ROFLMAO??

Lol @ the Guru!

Ha!!