Nikon D300 Complete Manual page 146

V1.02
Here's a piece of a JPEG shot with the lowest possible settings at a high ISO
value. Let's look at it a little more closely:
Looking at the edges of the letters you'll see slightly more and rougher
"noise" than in the open areas outside the letters. In particular, look
between the "p" and "o" and should see that there is more "noise" there
than you see elsewhere. But let's make it more obvious:
When I apply a bit of noise reduction in post processing you should see that
the edges of the letters are still suffering more from noise than the rest of the
image. That's because the JPEG artifacts buried at the edges of detail are
interfering with the noise reduction routine. Even if we get very aggressive
with our noise reduction we're left with obvious remnants of problems
caused by the JPEG compression:
Curiously, applying a small amount of "blur" to the original
data before applying JPEG compression reduces the visibility
Thom Hogan's Complete Guide to the Nikon D300 Page 146