4th D600, same problem.

Maybe you not know any dust is magnified viewing at 100% with a higher MP camera.

Whether you keep the inside of your body and lens cap clean, whether you point the camera down when changing lenses, whether you move the zoom and focus rings quick on some zooms, whether you work in dusty conditions and whether your camera is set to clean the sensor each time you switch it on all affect how much, or how little contamination reaches the sensor.

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Leonard Shepherd
Many problems turn out to be a lack of intimate knowledge of complex modern camera equipment.