It is possible that what you are hearing is bits that are usually contained in low level pasages of music were there are insuffiecient bit depth to dither correctly. (correct me if I'm wrong guys) This is the point of adding a noise to the recording such as apogee does during there uv22 process. When there are insuffiecient bits to dither down a noise is created, this can be a small pop or crackle, when you add almost inadible noise to the recording you are increasing bit depth, there for giving the dithering program the information it needs in order to process the sound. Try dithering in a program that gives you different options of ditheriing algorythms, to see if it makes a difference. You must however, start with the original file 24bit, once a file has been dithered you can not reverse the process.