When the slide is in, I'd check that the outside of the slide just to be sure that it fits snugly and evenly all around side of your RV. While the slide is in, make sure the slide has a clear path to slide out. i.e. look under, above, and on each side for anything that doesn't belong there.
One further thought. I had a similar concern on my sprinter. It took a while, but I finally traced the 'noise' to a bouncing cable located in the wall behind the driver's seat. Good Luck!