Rear windows making a popping sound when rolling up and down.
 

Both rear windows make that popping and slipping sound when going up and down. I assume there is some kind of gear that needs replaced. I saw a kit on ebay that is supposed to fix this but I want to make sure I get the right thing before I go on with this repair. Thanks.

door panels should be pretty easy to pull off (you're going to have to do it anyway to repair). Then take a look at window regulator components to see what's causing the problem. Otherwise sounds to me like you're flying blind.

It was pretty easy to remove the panels on my 300td. The problem was that there are no limit switches on the motors! So if you or some one else holds the button down longer than is necessary, the motor continues to try to crank the window either up or down. The regulator casting isn't very stout so it twists under that load and eventually the alignment between the motor's gear and the regulators is so out of whack that the motor strips the teeth off the regulator. My friendly local benz wrecker made some rude comments about how they are all like that to some degree and sent me a way with several good used ones and vague instructions to try to bend the regulators back into a semblance of flat before installing. The regulator casting is fragile (which is why he supplied several) so be careful! It doesn't have to be perfect, and will bend again when someone holds the switch too long, so be vigilent! Good luck.
Hahn Rossman
www.alkifoundry.com
1981 300td
1987 300td

I happen to have a similar sound in one or the rear windows on my 300sdl.
my problem is i am missing a plastic slide that fits into the metal track on the bottom of the window.
when you roll the window down it sounds like its missing a cog on the window motor...when you roll it up it makes a small clank.

I think hahn is all over your problem. They will twist, and they will also crack, and the gear mesh is off. It seems to be some kind of pot metal. My passenger rear regulator in the wagon was cracked and twisted, and made that sound. Took the one out of parts car, and it was worse.......