The stubborn black speck on the top right of the attached photo could not be removed by Singapore Nikon technician. The camera(Nikon D3400) is still under warranty. I was charged for the cleaning but found the speck was not removed. After they took it back to clean again, I was told the dust was “behind” the sensor and cannot be removed. They have denied it was a defective sensor and quoted me a new sensor for about S$ 200. They offer to waive the labour if I take up their offer during this warranty period. I am still in doubt of the reason given in regards the” dust behind the sensor”. I hope someone can tell me whether dust can get here? Could this be manufacturer defect?