Could it be something like a loose parking brake cable? Or (real stretch here!) a loose bolt and/or bad rubber bushing in one of the suspension arms - does the car handle OK? Is there any rattle noise in the doors when they are firmly closed?
Final wild shot - I noted on a stereo thread on this site that someone had put foam tube type water pipe insulation around the coil springs in the trunk. I tried this and an annoying rattle that occured when the trunk was closed went away forever! Good luck - maybe you could do a repeat of the parking lot ride-in-the-trunk with the person listening through a diagnostic stethascope to different areas to try and pinpoint the area. Might be more accurate than my amatuer shotgun diagnostic attempts!
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E