Tue 07-May-13 12:11 AM | edited Tue 07-May-13 12:14 AM by ScottChapin
I'm assuming that it is not in your pictures? Sometimes dust gets under the focusing screen. They aren't removable like they were in film cameras. You can remove and replace it but its fussy involving the removal and replacement of shims.
Cleaning the fresnel surface is delicate in nature as well. If it annoys you and doesn't happen often, you might have it professionally done too.
Scott Chapin Powder Springs, GA, USA Nikonians Team Member