In late winter and early spring male great spotted woodpeckers are out head-banging. You'll notice them especially in the early morning as they beat out a repetitive "rat-a-tat-tat" sound on a dead branch or tree trunk with their beaks.
This beak bashing is drummed out at around 15 beats per second. It's a kind of woodpecker 'Morse code' only much faster. The noise is telling other male woodpeckers to clear off, this is his space, and only interested female woodpeckers should approach.
Listen to the call of a great spotted woodpecker drumming - MP3 - 16 KB