BUZZING SOUND WHEN HALION SONIC IS OPENED

Iam using halion sonic from within native instruments maschine, on a mac. When ever i save & close a project, then reopen it, a buzzing sound starts, the only way it stops the buzzing is to reopen the project a few times or remove halion as a plug in from the project.
Does any one have any ideas to what this buzzing could be ?