Have a 2012 304d cr and the window rattling when tracking has gotten to a point where its almost unbearable and honestly im at times concerned about blowing out or cracking the glass. Not sure if anyone here has had good luck with an aftermarket seal of some sort or if hust calling the dealer is the best bet?
Which window? I assume you mean the front window, if that is the case, check the latches. The older C models had a latch on each side, they tend to wear out, and either not latch at all, or allow a lot of room for rattling. The D might have a top and bottom latch, I am not sure. Chances are that you rattle is more in the latches that the seal.
Was actually talking about the sliding side windows. The front glass actually stays in place very nicely.