A couple tiems a year I remove my roof rack and basket to give my roof a couple coats of wax. I blow the pine needles and dust out of the roof rails/gutter while I am at it. This time I decided to pour some soapy watter in each of the access holes and let it sit. With a strong stream of water I was quite surprised at the gunk and crud that came out. I am sure most was pollen and trail dust, but I still removed a few chunks of unknown bits. It took three rounds of this proceedure to get clear water. If you have never cleaned out your roof gutters it might be a good idea to help prevent corrosion.