Why does router send packets to the multicast address?
That's rather normal – many home routers come with IGMP enabled (both for improving local multicast, and sometimes even for receiving multicast IPTV from the ISP).
That means that one device in the network, usually the router itself, becomes an IGMP "querier" and every so often polls all other devices for changes (e.g. is the device still alive, and is it still subscribed, as IGMPv1 apparently didn't have an "unsubscribe" function).
Do not be unnecessarily paranoid about ICMP & IGMP.