[CentOS] Outbound traffic spike every 30 minutes

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?