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On a few older modems, some additional flashing is caused by multicast packets. Although these are usually filtered out, some modems flash their activity light before the application of the filter.
Once one has established their machine is virus free and firewalled, there is no reason to be concerned about flashing of the activity light. The downstream has *plenty* of capacity to handle this traffic, so it should not cause any performance degradation.
Most cable modems have an LED labeled Cable or Data or D/S, which indicates when data arrives from the cable network. Some users get worried that this LED flashes even when their PC is doing nothing or is switched off. If they are running a firewall, they are puzzled that the firewall does not register any traffic. There is no need for concern: some essential data (DHCP and ARP protocols) is broadcast into the cable network and arrives at every cable modem whether your PC needs it or not. You need not worry that someone is trying to hack into your PC just because the data light is flashing.
If the rate of flashing goes up when your PC is online (compared to when your PC is switched off or disconnected), but you do not know of any reason why your PC should be generating network traffic, then that might be a source of concern.