An intermittent crackling sound is the demo limitation for all our commercial plug-ins, except for Zebra2. After a few minutes of use, unregistered demo installations of our plug-ins will start occasionally playing back a vinyl-like static sound, roughly 5 seconds every 30 seconds.
Zebra2 uses a different demo limitation: after about 15 minutes of use, it will start playing quasi-random notes, instead of the notes you send to it. The notes Zebra2 plays are restricted to C-E-B-R-A, where R stands for a randomly chosen note.
The demo limitations can be removed by authorizing the plug-in installation with your registered name and purchased serial number.
See: Q: How do I remove the demo limitation?