Some links I found that appear relevant.

http://www.digitalhome.ca/forum/showthread.php?t=92697&page=32
www.digitalhome.ca/forum/showpost.php?&p=1252080&postcount=3

And then I found this link, in particular the "Random Beeps or Tones" paragraph -- I have that issue from time to time.

http://wiki.voip.ms/article/Improve_Audio_Quality