When I turn on the air conditioner in my car, especially if I’ve been driving for awhile, it smells like manure. Usually when this happens I’m not driving past any farms and I don’t live on or near a farm (I do live in a rural area though.)
In addition, I just had a friend notice that the passenger side vent is loose- you can kind of push it in and out, and I doubt it was like this when I bought the car. I just bought the car a few months ago from a subaru dealership.
98 ? % chance it is organic (like leaves etc.) in the area under the grill right in front of the windscreen.
Lean it out as best you can and then turn the vent fan on high and spray a can of lysol (or like product) through that grill. That will likely fix it. You can by a product make just for this use at the auto parts store, but it is only marginally better than the lysol.
Have you checked the cabin air filter? could be residue has built up inside the duct.