Audio tracks include audible artifacts such as "ticks", "pops", "crackle", "hiss", "censorship bleebs" etc.
Issue is most likely to affect older content. Review the issues and timecodes highlighted in the Redelivery Request. Seek an alternate source if possible. If no other source exists attempt to reduce the offending artifacts using any of the various digital audio software tools available. Should the issues occur over dialogue this may be more difficult to correct depending on your tool set.
If you are unable to improve the audio please contact your Netflix representative via Backlot outlining the challenges you are encountering.