firstly: please post such things at the bug reports forums, scroll down to see these 
didn't know it was a bug, i just asked before to be sure.
secondly: that's no bug as the output buffer is kept busy which is needed, f.e. for some plugins (e.g. a plugin that needs very long at startup to load)
i dont understand
i have no plugins installed and the note needs about 0.5sec to be played,
if i dont clic on the stop button during 10min, the cpu will work during 10min. why do i need to clic stop to have a normal load.
The sound processing that is going in does indeed take some cpu, even if nothing is being played. The reason is simple: once you play a note, the soundbuffer is set to open, but OpenMPT does not detect when this note stopped playing, because it could be a looped sound, or just a really long note. In fact, doing all these checks will probably cost more CPU usage, than it would gain if the audio was monitored like that.
I do not considder this a bug, but a side-effect of how the program works.