Lately we've been hearing in our garden in Cambridgeshire, from high in the trees, what can only be described as guttural glockenspiel notes - a bird impersonating a woodpecker in song. I can't get anywhere near it in Google searches or via your responses to other questions. No tune, no singing, nothing high-pitched, just gock-gock-gock-gock-gock. It's so unusual we can't help but think it's escaped from somewhere. Any ideas?