The CLS has a common issue with the window not sealing properly against the door frame. When this happens, water and dirt/debris is able to get in the cabin. But it also introduces a lot of wind noise, which is not something which fits the luxury of this car.
To fix it, you need to undo 1 bolt in the door. I believe its a 10mm bolt which enables you to adjust the tilt of the window. Mine needed tilt more to allow for proper sealing again. I used a 1/4″ ratchet with a long extension. Its recommended to remove the trim to see what you are doing.
The bolt is accessible through the round rubber grommet in the door, but is hard to locate without removing the upper door trim.
This is a great, free fix!
All CLS55 AMG pictures here in my Google Photos album: https://photos.app.goo.gl/6SP5PFAUXCqanoSX9