Duuuuuuuuust in the lens.
All we are in dust in the lennnnns (dust in the lens)
Sorry, what? Oh, right!
If you cannot see it in the pictures this shouldn't be a problem other than the simple fact that dust is getting in there somehow. Make sure to always have caps on both ends at all times at this point and if it is getting in somewhere else it's a fairly large problem.
If you can see it on the pictures you may want to send it in to Nikon.😱