Hello and welcome to our support group
Very interesting finding with the side skirts, but I'm not sure if you are describing the same issue we are having.
I know it's 20 pages but have you tried to browse through this thread? You can find the detailed description of the noise, it is very specific. So far we concluded it originates from the front, possibly the front suspension and/or brakes combined with M Sport package. But there is nothing that we are certain of, other than some users reporting that removing the strut brace makes the noise gone.
If you are certain you have the same issue your findings would be really helpful to us. I am also thinking about soundproofing the doors and all wheel arches but I wasn't able to find anyone who already did it. Was is relatively easy? Do you have any pictures from the installation?