So are u suggesting that its normal that D750 is producing or showing that noise and now I have to clean that by using the software ? then why are all people around saying that their images are clean at ISO 3200,6400 and even at ISO 12,800 ?
Of course, it is impossible to create a cam that will not have noise during low light situations. It's not the cam, it's the nature of photons.
Let me explain what happens.
What we call noise is nothing but an inaccurate signal. The sensor captures photons and releases electrons in turn. But because photon behavior is not constant and fluctuates, it does receive a variation of the signal it theoretically should receive. There is always noise present in any signal we capture and that amount of noise is the square root of the signal captured. If a sensor pixel captures 100 photons there is 10 noise. If it captures 10.000 photons, there is 100 noise. While the second has more noise, it has a signal to noise ratio of 100 to one while the first with less noise in total only has a signal to noise ratio of 10 to 1 which implies that result will be much noisier in our shot.
There's nothing the cam can do about that part; it's physics and unavoidable.
We can get around it by trying to collect as much light as possible during a shot but that is only possible by opening the aperture or increasing the duration of the shutter. ISO does not affect it since it only amplifies the existing signal and thus also the existing noise.
All of this does not even include any in-cam noise which can be added for several reasons.
Thus, during low light you'll always have noise and the only method in removing it, is by doing that in post.
If people say they have a clean signal, they shoot during perfect situations and either clean the rest in post or simply don't check the shot at 100% level. I never shot any high ISO with the D750 that doesn't have some noise.
Good noise performance only implies that it doesn't fall apart as fast as previous models. The D750 does very well but that's all you can expect.