Yes, it is internal issue, just google and read on D600 dust, then you will understand the root cause. Cleaning the sensor yourself will risk scratching the sensor as there might be metal dust on the sensor.
Yesterday, Came back from 10 days holiday to Shanghai and had taken more than 2000 photo with D600.
This morning went for marco shoot and found the sensors have more than 100 spots from photos taken with
f25.
This afternoon took a close up of the D600 mirror area and found similar scratch as...
1st time ( 3 days old) they said is dust & keep saying I change lens a lot of times. I only have 1 FX lens. 2nd time ( 10 days old) without saying anything, straight away bring in for cleaning. Both cleaning are FOC. Nikon rumor forums also mentioned this issue.
Always at top left portion. This morning after Marcos section, still got the same spot at same top left portion. Try shooting at f20 & above, then the spot is more visible.
My got oil spot again after this morning shoot at f25( Marco) even after sending back to NSC 2 times for cleaning. Funny NSC still don't want to confirm is oil stain and still say is dust. The spot is always found at top left portion of the sensor.