[CentOS] Outbound traffic spike every 30 minutes

Tue Dec 3 21:36:54 UTC 2013
Bowie Bailey <Bowie_Bailey at BUC.com>

Since Sunday morning, one of my CentOS servers has been generating a 
small spike of outbound traffic every 30 minutes (X:00 and X:30). It's 
not enough traffic to really cause any notice except for the fact that 
it is a very regular pattern and it started abruptly at midnight Sunday.

This server is used for mail (Courier-MTA), and DNS (Bind).  I cannot 
find anything unusual in either of those logs.  I tried grepping through 
my firewall logs, but have been unable to find anything useful there 
either.  I don't see any cron jobs that would generate network traffic.

Any suggestions how I can go about tracking this down?