I was able to visit Delta Ponds this morning and, as I walked along one of the ponds, I heard the loudest racket. It was so loud that I thought someone was using a Jackhammer across the river! But when I walked a little further, the sound now came from behind me! I began to suspect what it was then, and I was right! Some kinds of woodpeckers will bang on metal to attract a mate. I had just passed a street light, which are placed along the path for after dark. That's where it seemed the sound was coming from. Sure enough, I saw this Northern Flicker on top of the box-like metal making up the light enclosure.
This wasn't the first time I have seen types of woodpeckers pounding on metal. The same thing happened to me at Alton Baker Park where a Red-headed Woodpecker was banging on the metal direction sign at the top of a wooden post. I found an interesting article on the subject, entitled "Why some woodpeckers peck metal." You will find it here: http://www.berkshireeagle.com/stories/naturewatch-why-some-woodpeckers-peck-metal,413397