Cisco WAN :: 3560G Multicast Routers Bombarded With Data

We have a Lan with several multicast routers. Each router handles different multicast groups sent by remote sources to the hosts on the Lan. All the routers are connected to a Cisco 3560G switch. The routers range from 7204s to 2621XM, and even a 6503 switch. We found that each router is constantly bombarded with multicast data from each other router, even though the router did not join any groups. Obviously, we'd rather not have this happen, because it causes each router's CPU to rise to very uncomfortable levels. If I bring a new router onto the Lan, as soon as I enable multicast, the data starts hitting the router's interface. In an attempt to stop the traffic, I protected each router port, so that (supposedly) the router will not be able to communicate with any other protected port.