I have a 2020 C63s,9400 miles. It had a horrible rattle noise when accelerating around 2500 to 3000 RPMs.It was the worst in S+ mode. It drove me crazy for about 2 months first we thought it was an engine mount or maybe a heat shield it was none of those. I found on MB forums w205 owners reporting the actuator for the H valve going bad causing the rattle. Sure enough I told the tech about it and he found it right away that was the problem He checked this by disconnecting it then driving confirming the rattle was gone. The dealer calls this the exhaust actuator/diverter assembly.
He also removed the center trans support and re torqued it down because that was rattling as well. To fix the actuator the tech removed the exhaust system removed the center flap actuator then clocked the spring and additional 25%. Reassembled and the rattle was gone after road testing. It was intermittent until I tunned my car to stage 1. I'm glad I did it because it was easier for them to duplicate the death rattle with a tune loaded. Hope this helps someone I was going crazy from this rattle.

 

Very common: https://mbworld.org/forums/c63-c63s-...63s-coupe.html

not just the C 63s, even AMG GTs have this problem.

https://mbworld.org/forums/coupe-roa...-roadster.html
https://mbworld.org/forums/coupe-roa...st-rattle.html

 

Same year and with 7800 miles and dealing with same issue as you. I sprays thick lithium grease into the flap housing and 3 weeks in the rattling hasn't come back. Ceramic coating the rim helps a little but still have put effort in cleaning them so I am investing in some actual ceramic brakes I found on a wrecked C63S.

 

Thats interesting I'm wondering what happens when you remove the flap. I don't think aftermarket exhausts come with the center flap just the 2 rear flaps.
I kind of want to remove it not sure what will happen.

 

If you don't mind having the valves open all the time, you can defeat the actuator using a bypass plug (you need two): https://burgertuning.com/products/va...r-bypass-plugs

Or you may also use an EVM (exhaust valve module and just keep it opened all the time): https://asr-component.de/gb/begin/82...-ml2-c205.html
For the EVM however, you will need to find a forwarding service to ship it to North America as ASR does not ship to North America.

 

These modules are the reason the flaps rattle.....well for me that's what's happening because if I set back to stock and open the flaps using the standard exhaust button there are no rattles coming from the 3 and sounds so clean. I tried thick lithium grease that stopped the rattle but it dried off after a few months so now gonna try thick aerosol graphite grease

 

I see, never mind the recommendation then.