Try checking your A/C drainage, also the drainage from your moonroof (if you have one). I haven't had this issue with my Subarus but it sounds a lot like when algae built up in my drainage tubes in another of my cars.
A/C drainage lines most likely culprit. You need to get under the vehicle and either blow them our our use a wire to snake them clean. Stay out from under it when you do because when it lets go there could be a mess.
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