NIkon has finally responded to the ongoing D600 sensor spot issue by releasing an official service advisory and offering a free shutter replacement to owners of D600 cameras:
Nikon is making available to all owners of D600 cameras (even if Nikon’s product warranty has expired) this customer-service measure, which includes the inspection, cleaning and replacement of the shutter assembly and related parts of your camera, FREE OF CHARGE as well as the cost of shipping D600 cameras to Nikon and their return to customers. Once again, please understand that regardless of this service, your D600 camera as is the case with all D-SLR cameras, will continue to require normal periodic sensor cleanings.
Wanna know how the new Nikon Df shutter sound compares to other cameras? Check out this test that compares the Df (regular mode and quiet mode) against the D4, D700 and Fuji X-100s.