I had a similar problem while shooting at a white piece of paper
indoors.  The paper has graduations on it that I use to establish the
focus of the camera by shooting at 45 degrees to the paper.  Anyway, I
solved my noisy image by setting the white balance properly.

There were several clues to the problem.  First of all, the white on
the image was not linear and appeared noisy.  It had a sort of orange
tint as you moved away from the center.  The histogram was mostly to
the left when it should have been mostly to the right.  The focus
wasn't as sharp as it should be.  The black lines on the page were a
good contrast in the center of the photo but appeared almost brown
instead of black as they moved outwards away from the center of the
photo.

I was a bit confused at first as I am relatively new to photography.  I
started playing with the image in Photoshop CS2 and Nikon Capture 4.2
until finally I saw that it was the white balance that was causing this
problem.  Once adjusted correctly, the histogram was in its proper
place and the focus of the image was much sharper, the black lines on
the paper were proper and the noisy white paper was nice and white like
it should be.

-----------------------------------------------------------------