Long exposure noise reduction is really just a name for dark current subtraction.
Your noisy images are normal and nothing to worry about, just be thankfull that your exposures are only 30 seconds.
There are two noise issues that need to be looked into.
When you do astronomical photography, your exposure times are up in the high hundreds os seconds, 300, 600, 900. This introduces huge numbers of noisy pixels (thats why most astronomers use canons, but thats another issue).
Astronomers will take dark frames (cover the lens cap) and image for exactly the same length of time. Normally 10-15 darks are taken on average. This image records all the following noise data.
1) Hot pixels.
2) Amp glow (heating effects due to camera electronics. Long exposures in summer are noiser then the same length of time in winter!!!)
3) Readout noise.
we then take bias frames. These are images of the readout noise which appear in EVERY photo. These are the fastest shutter speed your camera does (with lens cap on) again between 10 - 15 are normally taken.
Then you have to think about flat frames (dust off images)
So your 30 second image contains the following data
1) Light image (what you can see)
2) Hot pixel noise
3) readout noise.
The noise reduction in the nikons only address the hot pixel noise not the readout noise. Plus the fact their raw images are not true raw!!!
You can use software to combine your images to remove the noise and end up with a true noise free image.
The reason for multiple dark & bias frames is to remove the random noise from high energy particles striking the sensor.
If i get time, i'll post examples