This post along with a video I found helped me out a ton with this issue. Hoping it's gone for good, or atleast for now and if it happens again I'm gonna do the exact same thing.
Had the same issue, sloshing sound from my passenger side, that weird hissing noise ONLY when I turned left with my AC on from my passenger side. I first thought something was hitting against my tire as I turned. Thankfully found this thread when I googled it.
Used a can of compressed air from Walgreens
Got to the hose, removed it from the nipple, blew air down the white nipple peice and nothing came out from under my car, then blew air up the black hose. Got a few drops, blew some more air and then the water started coming out like a faucet.
Got about half a gallon of black water out of there, maybe a bit more. Had little debris in it, a couple little pieces of leaf twigs. Sprayed more air and nothing came out.
Hoping this is fixed, especially that hissing sound when I turn left. If it's still there imma guess I'll need a new fan motor or something that I can't do myself (not a mechanic) but hopefully that's the end of it.
Going to place a new cabin air filter in aswell. Haven't updated that since I got the car December 2018(2011 Ford edge Limited)
Seriously though, really appreciate this thread. Guarantee a mechanic would've easily charged me $100 to do this.