What is this noise pattern?

I was trying to stretch out the maximum amount of nebulosity from this single frame (i.e. not stacked) and a geometrical pattern emerged in the background noise. Has anyone seen this before? Is it a property of the camera sensor, or does it result from within Photoshop, which I used to stretch contrast? I'd be pleased to hear from anyone who either knows what causes it, or knows how to reduce it in processing. Meanwhile I'll be stacking several images with darks and flats etc., which does seem to get rid of it.

CMOS sensors suffer from pronounced fixed pattern noise. You'll need to dither your exposures, this will allow you to avoid a succession of exposures that occupy the same pixels. Using your mount's hand controller, shift either the RA or DEC slightly after every few exposures. I press the button 3 to 5 times. This is assuming you'll be taking many sub exposures in the stack. If you're only taking a dozen or so then you may want to dither every other exposure. Dithering will render the outer portion of your stacked file unusable due to the stack alignment but a slight crop will remove it.

Thanks Steve, very informative.