Sounds are made when objects vibrate. If there is a void under that piece of trim it will amplify the sound.
My thoughts are if you can access underneath that trim, and if it is a void, you might be able to stuff in some sponge to deaden the sound. I am sure someone will come along who will know what is underneath or how you can access it. Just my thoughts. My leather seats rub and make a noise - because I know what the problem is, it does not worry me that much. When you can't find the issue it does tend to drive you crazy!