Are you talking about specific traffic, to a specific bogus IP address? Or, are you seeing high volumes of ICMP pings to addresse not on your network?
Maybe you have one of those worms that's been going around the Internet, Blaster or Nachi. Symptoms are high number of pings, to sequential IP addresses; also connection attempts to other machines on TCP port 135 (Microsoft uses this port for Remote Procedure Calls), again with sequential IP addreses. I had a customer with just a handful of infected machines in their enterprise; traffic from them essentially shut down all routing between VLANs at their core.
Had to use a Sniffer and do some packet captures to identify the offending machines, then isolated them; and then cleaned them with utilities downloadable from the Internet.
Hope this helps.
