[CentOS] OT - Is there a package to monitor network traffic

We have a situation here that is a real mystery.

Our MRTG on our outgoing router  and a firewall server that protects our 
web servers is showing a spike every six hours. I can't find the server 
behind the firewall that is generating such an extreme amount of 
packets, even though I've looked through the crontabs of nearly all 
servers, performed "ps" variations, and other types of investigation.

Is there any type of package I can install that will monitor traffic and 
report abnormal, over-threshold packets similar to what wireshark might 
do in a manner that would allow me to determine where these packets 
might be going or from where they originate?

Thanks for any help.

steve campbell