try do ur test shot at f20 and above.

more or less, whether ur lens or ur sensor, most of the time some tiny dust will be there.

i blow my sensor and mirror, but one time i blow it with a wrong angle, the dust fly inside the penta-mirror and stay there. Nikon says need RM200+ to remove it, even still under warranty..

blow directly to the pentamirror, which means the area above the reflection mirror,
this will push the dust to fly inside to the pentamirror thru the small gap near the reflection mirror hinge.