I've recently come back from a caving trip where there was quite a bit of (surprise surprise) mud and dirt around me. While I did try and take care of the camera (I carry all my gear in Pelican Cases) as good as possible my fingers were unavoidably dirty and some of that dirt appears to have gotten into the Lens' focus ring. When manually focusing now, it makes a scratching noise under the rubber ring. I took the lens to a camera shop nearby and they suggested that since no deterioration in image quality was apparent and the function of the lens was not reduced, that there was no need to open the lens to clean it. I would, in future, like to sell this lens and replace it for one of the new 24-70mm canon lenses that have come out recently however i believe this would make it more difficult to sell. Is it possible to clean this? Do i need to get it professionally serviced? How much is it likely to cost?
Hope you can help.