As a logger by trade, I have considerable experience with woodpeckers. Kind of a woodpecker myself I suppose. I think the pecking on non-food objects is their version of a territorial message to other woodpeckers. Most birds voice their territoriality by song but most species of woodpeckers are not very vocal. Hence they use their pecking sounds. If you listen to them when they’re pecking on metallic objects, their pecking is often a repetitive sequence of x number of taps followed by xx number of taps, etc. You can memorize the sequence and mimic it by tapping on a metal object yourself. Sometimes they will answer in an agitated manner. I think it pisses them off.