I'm with Greg; my D600 will be going to Nikon LA for a "sensor cleaning" before its warranty expiration. I have previously cleaned a D200 & a D7000 sensors with no problems but my D600 has issues. I now have filaments (red, white) sticking-upward from the bottom of the shutter box that I must tweeze-out during and after cleanings. And for all my cleaning efforts, I still do not have a clean sensor nor clean shutter box. About six months ago, I stopped changing lenses and only keep the 24-120/4 on-camera; I do wipe the barrel of this lens to avoid the "bellows-effect".
My D600 would already be at Nikon Service but we have an upcoming trip to Saguaro NP, White Sands, and Carlsbad Caverns.