I have the same sound (pretty loud at times) and it seems to be coming from the front of my large front slide. I tried lubricating the gasket and it did not help. I am now leaning towards the slide cover stop on the front. It could be too loose or too tight. I have not tried adjusting it yet, but when I get home from this trip, I am going to mess around with it.
Here is a picture of the same stop on the rear slide. It shows a little better because of the lighter colors.