i recently bought an f75 and gave my id edge to my father, anywayu on my f75 when i go to discrimination mode and put notch on 15 and discrimination mode on 20 i am hearing like im in all metal the tones that are supposed to be silenced are coming through as a muffled low sound so i am hearing all targets its very aggravating i love the detector but not sure why this is happening is it something im missing. please help. thanks shiloh