I'm seeing the 'noise' predominantly in the circled areas below...
First of all. Make some more tests.
Take a photo of a pure white surface, a piece of new paper, take it out of focus and take a look. Try to isolate some other factors, like the time you are using the camera (this can heat the sensor) or room temperature.
There is a chance something is happening to your sensor, probably some overheating.
(I have a similar problem with a video camera. I'll make some tests too)
(Offtopic. Probably you are pushing too much magenta into the processing, therefore exaggerating a bit a magenta cast)