I have not checked whether the horsehair (or other insulation material) is still there, and Rainmaker, I'm very sure it's not water that is sloshing, it's fuel!
I guess I'll go stuff some insulation material in the back seat, and if the sound still persists loud & clear, I'll just leave it as that. It sounds embarassing, but if the car runs fine, I suppose I'll resign myself to it.
Thanks for your comments!
1983 W126 (280SEL)