How about option that renders 1 track at the time? I know theres option that renders 1 track to different files, but I mean that rns renders 1st track to wav, then starts over again and renders second track and so on.
I have noticed that if cpu usage is near maximum, existing render options cause crackles. And when I render one or two tracks at time, crackles disappear.

Rendering shouldn’t cause crackles. Rendering should simply take the
time needed to finish, there’s no realtime world to consider. Do you use
any VST’s that read from disc? 1.5 has some more rendering options,
but not this one. You should test this during the upcoming alpha/betas,
and see if it’s still a problem.

No, but I think the problem is on ReFX Vanguard. Got to do some research.

The point is: using CPU power doesn’t magically add crackles to audio. :)
When rendering there is no real time to catch up with, and therefore there’s
no reason to have crackles since your CPU can just spend all the time in the
world to finish the rendering without any soundcard nagging for more data.

I have this problem too, but then i reboot my machine and it appears to render perfect… it seems.

renoise fails to render some tunes too, this is rare but it happened to me two times, really crazy actually it renders silence for the whole song and about 2 secs of nonsense in the end, or feedback through the whole tune… if the renoise developers are interested i can put up the .rns somewhere …