These are usually caused by 'buffer underruns'. That is, the audio stream to the soundcard is interrupted when a temporary memory-buffer holding the audio data runs dry. This is caused by CPU and or Hard Disk overloading. Crackles and pops can also be caused by plugins behaving badly due to incorrect settings or design flaws.
To learn more about resolving either or both these issues see the manual sections on...
1. Buffer underruns - In short, select an ASIO audio driver on the Audio Settings page (Options > Audio Settings) and or increase the Buffer length setting.
2. Plugins behaving badly - In short, open the plugin wrapper PROCESSING tab and deselect 'Allow threaded processing' and or select 'Use fixed sized buffers'. There are more options to try but these are the most common issues.
3. If you hear an intermittent hissing sound (white noise), then this indicates that a plugin is demo mode. Check this article.
Image-Line Support Team