computer help

hey on my computer like every few minutes my computer makes this errrrrrrrrrrr sound then it slowly gets softer and softer then gone and it does this like every few minutes, do you know what this is or is it a problem? also i was wondering about the router firewall, does it automatically enable right when you just plug in your cable wires and stuff and the firewall is already there or do i have to do something to make it work? i have a netgear router