First, turn in-camera noise reduction off. You can do a better job of reducing noise in PP.
Second, two factors tent to effect noise. The first is ISO. Of course, the higher the ISO, the more noise you get. Since you are shooting outside, on a tripod, you might want to try reducing the ISO for these shots. I'd keep it below 800.
The second factor is exposure. The more light you let in, the lower the noise. You might want to try exposing to the right (ETTR) here and bringing the exposure down in post (as a batch process). That should reduce the noise, significantly.