I discovered horizontal and vertical noise patterns in the dark areas when zooming in on the image
That pattern in inherent to the sensor and the way Canon cameras record the raw data (the same if it gets converted in JPEG in-camera), but normally it "falls under" the black level, i.e. it vanishes in the darkness. When you have to increase the intensity (for example by the "exposure" slider in ACR/LR), the pattern appears.
The appearance of the pattern depends on the way raw the raw processor interprets the data: it may be dominantly horizontal or cross like.
See the attached images; the first one is from an ISO 3200 shot of Ray; it demonstrates, that the pattern depends on the intensity. The other is from an ISO 100 shot of a black sheet, the raw channels are shown separatedly, you can follow the pattern. This shows, that the pattern does not depend on the ISO but on the pixel intensity.
So, plainly, it is the too low exposure. If there is no danger of clipping of important details, use higher ISO, not instead of the long exposure but in addition.