I just wanted to repost this excellent article on causes of image noise from DPReview.
It’s frequently drilled into our heads that image noise only comes from higher and higher ISO settings but you can actually get noise in other forms. Any low light situations can have noise even at your lowest ISO settings. The basic takeaway is if you’re post processing, expose your images to the right without clipping off highlights and then bring the exposure back down in post. You’ll end up with better detail in the shadows.
It’s easier to bring down unclipped highlights then bring up near black shadows, there are a lot more photons to work with on the right end of the histogram.