Not sure on your sloshing sound.
Did you pop off the rear storage carpet threshold cover and feel under the rear carpet? It's not uncommon for leaks to develop (at least on mine) along the roof rail seams under the roof rail covers and trickle down the rear corners on either side of the rear cargo door. The foam below the carpet can absorb an incredible amount of moisture. That wouldn't cause 'sloshing' sounds, but if water worked into another area and didn't drain out, that might.
The rear carpet on mine was soaked last summer. I battled such a rear roof leak for a quite a while till I got it resolved. When I removed the rear threshold, carpet, and the rear side panels (they all 'pop' off pretty easy with just hand pulling), and put a hose up to the roof, there was a steady drizzle of water down the side of the rear roof (near the rear cargo door) into the cabin. You wouldn't see it with all those plastic panels installed.
I suspect rear cargo door water leaks are going to become more common as these rigs age, for the following reasons:
1) Factory seam sealer on the top rear roof pinch weld area (where the top rear weatherstrip goes), the seam sealer can crack or rust in the seam on the top corners. Even so, there's a height difference there between thicknesses of metal.
2) The rear weatherstrips have metal in them that 'clamps' them onto the pinch weld. That metal can rust in rust-belt areas or just become stretched and weak overall and not pinch tight to the seam. I re-bent the metal tighter with a special (wide) metal-forming pliers which made a big improvement. It fit a lot better but on the top strip it didn't fit tight enough and replacement of that piece was the solution. But it made a big difference in the perimeter gasket which goes around the cargo door.
3) The top roof edge seams under the plastic roof rails are eventually going to crack at the seam sealer, or leak where the bolts or clips go.
I worked at a body shop for years and after a while you can just tell how things are put together are going to eventually rear their ugly heads and become problems as vehicles age. It's normal and not new or specific to any vehicle.
Back to your water sound, the rear carpet threashold cover pops off nice and clean (shouldn't damage the clips). If your carpet foam is nice and dry that probably isn't your problem. I had to remove my top outer plastic rails and re-seamseal the outer seam. There I just used a tan urethane caulk that sort of matches my rigs paint color. I also had to replace the top rear weatherstrip that attaches to the cargo door roof ....and apply some non-hardening caulk on that roof pinch seam as well as the seam for the weatherstrip surrounding the rear cargo door area. Car mfgrs commonly use a non-hardening bedding compound (and have since the 60's) which is this stuff if you want to be technical. It's messy stuff but cleans up pretty well with WD-40.
https://www.3m.com/3M/en_US/company...Glazing-Compound/?N=5002385+3293194048&rt=rud
Anyway, that all did the trick for me. The problem with 'letting it be' if one has such a leak, is it will eventually trickle down and soak the rear pass footwell carpet area and rot the floors.
