It will be power saving, buffer settings, mismatched sample rates between Audacity project rate and the Line-In in Apple Audio MIDI Setup, or all three.
See http://alphamanual.audacityteam.org/man/FAQ:Recording_-_Troubleshooting#mac_crackle. Don’t look elsewhere in that Manual because it is our development manual. After Audacity 2.1.3 is released the link will be http://manual.audacityteam.org/man/faq_recording_troubleshooting.html#mac_crackle.
Gale