My guess is it has to be something shedding from the shutter, my second d 600 is a mess just like my first one, after about 800 shots there are hundreds of specks on the sensor. I bet if you could count them it would be 1 for every shutter click. Anyway I have only had one lens on the camera so it was only open to the air for a few seconds. Also when I checked it the first time there was one rather large spot, now it's covered. Photo in my gallery.
Nikon is being totally unresponsive to my e mails I am calling them tomorrow. My vendor will take it back but they have a meeting with Nikon at a photo show this weekend.
I am really P***ed that they won't even acknowledge that there is a problem. There are plenty of reports out there. That is just bad business.