A quick update to my problem.
A friend of mine drives a 1995 Nissan Sentra 160GX.1600cc 16valve carburator engine.
I do not know if the name if these nissans are used there in Australia, but the main thing is that it is a Nissan.
The weekend we drove and I mentioned it to him about the datsun.
He said "Lets see what mine does".
We then drove in second and he flored it to 6500rpm where his car's redline is.
I think this small vibrations that I talked about was as Dodgeman mentioned harmonic frequency.Just as we reached 5500rpm it got exactly the same vibration and little harmonic type noise fro the engine as I got at 5600 to 5700rpm.
Although his was a little bit quieter than mine. But, remember.
Its a couterweight crank and much more damping material than a old 1977 datsun body.His disappeared at +-5900rpm and then the engine sounded asif it wants to climb out.But again not so loud as my datsun.I think this is just a Nissan/Datsun thing and that I placed too much emphasis on it.The vibration I think is the flywheel that I am feeling through the gearlever when reaching 5800rpm.
Thanks for all your replies.
But I think I got an answer locally.