Hi Stevie1982, I've just registered with the forum because I have been trying to find an answer to this for over a year (including finding this thread last week) and finally solved the problem yesterday. It's fair to say the rattling has been driving me up the wall and as I've found the solution it seems only fair to pass it on.
It seems that some people have managed to solve the rattle by wrapping duct tape round the bar on the seat back which goes into the catch - it didn't work for me.
This is the exact rattle that I had:
Wagon rear seat rattle - MBWorld.org Forums
There is a related thread on that forum:
S212 rear seat backrest removal - MBWorld.org Forums
And at the bottom of the thread is this advice:
@carlsbadbill I had the exact same rear seatback rattle you describe in my 2016 E350 wagon and was able to fix it myself. I'm not very handy and was terrified to ruin my seat but this turned out to be a simple fix.
For me, the sound was most easy to replicate if I hit the front of the seatback firmly near the armrest. It was kind of a rattle accompanied by a short metallic buzz. I wish I'd taken some photos but here's a brief description:
Fold the seat back down. The carpeted, rectangular floor piece in the cargo hold directly behind the seat can easily be lifted out to provide a little extra work space. Pull firmly up on either side.
I did not remove the carpeted seatback. Instead, I detached the fabric at the bottom of the carpeted seatback just enough to reach my hand in. Use a flat head screw driver to gently push down into the seam and then pry back. Start near the corner and work your way across the bottom prying every 3 to 4 inches. You will begin to expose a soft, plastic "U" channel that is affixed to the fabric. Don't pry any one position to far or you may crease the plastic U channel. Also don't worry, this reattached easily for me.
Once you have opened this seam (at the bottom only) then you should be able to reach your hand between a metal plate (essentially the carpeted the seat back) and the seat foam. Near the center, just a few inches inside you will feel a metal rod. You can "pluck" that rod and notice that it really vibrates. I agree that the sound seemed to come from the center of the seat, however, I jammed a microfiber towel between that rod and the metal plate very close to the bottom of the seat and this eliminated the noise completely.
To close it back up, just push the plastic U channel back under the metal plate. I was able to get this seam closed 95% by hand and got it factory tight with a few taps by a rubber mallet.
I was prepared to be frustrated but this was all very simple and I only wish I'd done it sooner. 20 minute job.
I followed this advice, although used a plastic trim removal tool rather than a screwdriver to avoid damaging the leather or plastic U channel which is pushed into the join as described. You don't need to push the tool in very far to be able to lever the U channel out - once it starts coming you can grip it tightly and pull it out further with your fingers if you are worried about the levering damaging something.
I needed to prise out the whole bottom seam to allow enough slack to get my hand into the seat back. It's quite tight access - you need to push in between the foam of the seat material and the metal backplate which the carpeted section is attached to. You'll feel the rod which runs about half the width of the seat. I stuffed a couple of microfibre cloths between the rod and metal plate using the trim removal tool as its probably only a 5-10mm gap and is tricky with fingers while also holding the foam out of the way. Refitting the U channel back into the seam join was pretty easy as described above. It could be pushed in with my thumbs and although I did tap lightly with a rubber mallet it didn't really need it. And it was indeed only a 20 minute job overall.
And the best bit - I took the car for a drive afterwards and the rattle is gone....OMG, bliss!
If it helps to see pictures of what you are dealing with inside the seat, this thread where someone replaces their seat back with a ski hatch version is useful. The 5th picture down shows the metal seat backplate and the rod you are padding is at the bottom of the picture.
W212 AMG - Ski pass-through install - All E63 sedans did have the ski pass-through. It was not an option on the E63 wagon in the US and very few of the E350 wagons had it. It was not a regularly ordered item for dealer stock cars, just special order cars may have it, but again only the E350. For...
mbworld.org
I hope that helps and good luck with the fix if you attempt it!
Ben