Hi I don’t know much about the 900 or even the AWD cars but to me it sounds like a place where water has collected.
In my past refrigeration career it’s common for it to happen in evaporators.

Maybe the AC and the HEATER core plenum is not draining itself of its condensations.

The air conditioning comes on in defrost mode on newer cars automatically.

Maybe if you felt around under the car for the duck bill rubber drain hose you can give it a squeeze.

It’s very possible that leaves or algae has gotten it plugged up.
When the fan runs, even at its lowest speed by default, it can whip up the water and muck.
You might try pulling the fuse to the fan to stop that idea.

This duck bill I’m speaking about usually drips right onto the transmission. The duck bill fits down through the body of the car on the 240s as I remember and can be reached from under the catch pan inside the car.
Or it is from above the transmission.
Been a long time since I have been under my dash for AC issues.

I hope it stays that way.

Phil