Digital BW, The Print

I had a "grid pattern" appear in a print on my R2400 today, and was at first worried
about the printer but soon realized it was just noise in my blue channel. Not 
sure exactly where it came from, but it's there in the file. Has anyone else noticed this 
grid-like posterization in darker areas of the print? (this print happened to be color, but if I'd 
used the blue channel in a B/W results would have been similar).  The original file was a 
Canon D20 Raw capture, imported through ACR as a 16 bit file and then adjusted minimally 
in Photoshop (can't see any real gapping in the levels histograms, though the bottom part of 
the blue channel is a little bit stair-step looking). The image was dark to start with so this 
may just be telling me to make better exposures -- that I'm pushing the limits of exposure 
latitude -- but I'm curious to know if anyone else has seen this particular phenomoenon or 
knows more about it. 

Linda