From the door latch area? A clicking may occur from the striker plate, there was the addition of a plastic strip on the inner edge of the striker, that would be worth checking.
The suspension spring noise, maybe a snapping/clicking noise? 2 other common problems with the W140 chaissis. First the sway bar links get loose. And the upper control arms making a noise on bumps. If the rubber bushing is smooth, you have an early one, the later ones have a single rubber bead that goes around the bushing...
Good Luck, noises can be a pain, be patient!!