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The lower your ISO is, the less noise will be in the image.
There is no such thing all sensors produce noise - it just a question of whether that noise is intrusive.
A few things you can do:
1. As mentioned by Szalam , keep the ISO low - this means that sensor noise is not being amplified
2. Get the exposure right. If you are using Photoshop or similar, to compensate for poor exposure and to increase shadow detail then along with that in computer exposure increase comes increased noise.
3. Shoot RAW. Most modern cameras shoot raw using 14 bit date (some are 12). Shooting raw allows you to use the image data in all 14 bits. Shooting jpeg restricts your image to just 8 bits. Not bad in itself but if you later increase the shadows - you will bring up compression artifacts.
4. Use the noise reduction in Lightroom or ACR
5. For extreme long exposures, take multiple versions combine them in Photoshop by stacking and then use the mean/median functions to improve the signal to noise.
I hope that helps