Maybe driveshaft center bearing or exhaust pipe/muffler hanger? Or a loose belly pan? Check the easy ones first: nothing in the center console or glove box that is loose, seat belt not banging against the center piller, wiring under the seat loose, etc. These noises can be hard to pin down.
I had the same kind of rattle - clink noise, and it turned out to be a pen that the previous owner had let drop down the defrost vent and it had worked its way into the ac ducts under the seat - it still had the owner's name on the pen!
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