By crackle I mean a short (shorter than one second, but longer than just a click) and rather loud fragment of noise (I can record it and post the waveform later). It isn't registered by Sonar's bus meters, but is visible on the soundcard's indicators. It happens right after I open the project and before I even start the playback. As I wrote before, during the playback there are no dropouts and everything works perfect. The only problem is this initial sound. Other audio software doesn't show any similar symptoms.