Likely an (older) ELT, probably activated by mistake, but you never know. Are you also getting a signal on 121.5?
If you're in a dense area, it's unlikely that there's a plane crash that no one else knows about. But if it;s a false activation, that can interfere with legitimate SAR activity if there is an emergency. So it's worth reporting. It's not a bad idea to call the nearest Coast Guard office and let them know what you're receiving and where you're located.