I particularly shoot images during morning or evenings as those conditions are favourable for landscape in my region, but while retouching the raw files in lightroom, I came across tons of digital noise. The ISO settings of my camera don't go over 400, but still I find lots of those pixelated noises. Is it the lens that is not working well or are there any proven techniques to reduce digital noise while shooting and processing?
ISO 400 can contribute significant amount of noise (as answered by null). I am assuming you are a beginner and answering it in a very basic way.
While shooting use low shutter speed (you may need tripod for this) and large aperture (least f-point available while shooting) to avoid high ISO value.
While post processing in lightroom, try increasing luminancec (for luminance noise) & color (for color noise) value under Noise Reduction option.
below is the link which explains how perform noise reduction while post processing.
Hope this helps.
my camera don't go over 400, but still I find lots of those pixelated noises
It looks like this statement assumes that 400 is a low ISO value in some absolute sense and thus there should be no noise.
This is not true at all. Noise is mostly relative to the ISO range provided by the camera. The closer you are to the maximum value, the more noise you get. You cannot tell absolutely how much noise should be visible at ISO 400 because that depends on the camera. For some cameras that would be the highest setting, thus they would produce a lot noise, for others it would be a very low setting with only little noise.
As rafal suggested, using luminance and color noise reduction will help, but there are potentially two other things going on.
1) If you are exporting to JPG, you can expect to see some increase in pixelation, particularly if you set the output quality lower (more compression). Be sure that you set the quality to 100%.
2) I think there is an option upon export for additional sharpening, beyond any sharpening you did during post processing. That can also increase the appearance of noise, again possibly exacerbated if you output to JPG at lower qualities.
Just to note: this is not so much a lens issue as it is a sensor/post processing issue. Smaller sensors often have smaller pixel pitch (size of photon-sensitive buckets), and so capture less signal. Less signal, lower signal-to-noise ratio (generally), and so more noise in the final product. Post-processing can either alleviate this or make it worse, depending. Often, this is only a problem if you zoom in to 100% or crop quite a bit.