I discovered some spots on my photos. They are not so obvious when shooting at larger apertures but at f11 they are visible. I tried to clean the sensor with all conventional methods but the dust particles can not be cleaned as they appear to be behind the front glass, on the sensor itself or on another filter inside, part of the sensor assembly or on the inside surface of the top glass(IR filter?). Anyone with similar experience? What could be the cist of repair?