This is just a wild guess, but perhaps you've got insects in your gutter and it's trying to scare them out so it can eat. The main reason the go pecking in the first place is to find bugs to eat. He probably scared up something from there once and just keeps returning to do it again. You could try cleaning out the gutters and making sure there's nothing there to satisfy it's pecking.
It is a signal to other woodpeckers that he is ready and available for a mate. The bigger the sound the better.
There is a woodpecker that has recently decided to take up residence on the side of my house. But wait! It is not pecking wood...it is pecking at the metal gutter! Every morning. Early. Very early. Aside from all the obvious issues, why does it continue each morning to peck against metal? Is something wrong with it? There is a perfectly good forest nearby.
