Another knocking noise, with a very hollow ring to it, seems to be caused by birds in the guttering (hence the echoing effect). I've only seen their shadowy outlines (through the knobbly glass of the bathroom window) while they were flying up there. So I don't know what species they are or what they're after. However, I'd say it was too much pecking for them to be trying to drink water and more likely for them to be seeking insects in the crud and moss.