I was not aware that the topic really gets closed, so I just open another one 
Old thread: Crackling at ~50% DSP load, but no xruns
Finally I found out that the crackling in Renoise (without showing any xruns) apprears only with the RT PREEMPT kernel.
When I play the project (which is a 16 line long loop) with the RT PREEMPT kernel booted I get constant crackling without any xruns.
When I use the stock Debian kernel I get xruns only during the first loop and then they disappear. So the DSP load is higher at the beginning and then goes down again.
Are usually unsing RT kernel when using Renoise in Linux? Officially it is recommended, but in my case it makes more problems.