You don't mention what vehicle you drive, but, I have noticed the same sound from my 85 300D. In my case it became apparent only after I cut a hole in the rear deck to install a subwoofer. The diesel has this strange plastic tank, which is only an inch or so thick and lies horizontal to the rear deck, it's not the main tank.
I've never bothered to find out what this is, but my sloshing sound comes from it.

My solution to the problem...crank up the tunes.

__________________
Ed "Don't Benz's just feel better..."

Current wives...
2000 ML430 Skyview "The Mel"
2000 CLK430 Cab "The Cab"
85 300D Turbo "The Diesel"

Past wives...
92 300E
85 190E

"One should as a rule respect public opinion in so far as is necessary to
avoid starvation and to keep out of prison, but anything that goes beyond
this is voluntary submission to an unnecessary tyranny, and is likely to
interfere with happiness in all kinds of ways."
Bertrand Russell