In reply to TeeJay:
Could it be mechanical? Some kind of wind caught rotating machinery or a rusty pump causing an annoying intermittent noise?
2013 photos & vids here
eff37 on Flickr
In reply to WendyBartter:
Hi and thanks for all your comments. Yes, this may remain a mystery and think we can probably rule out the possibility it could be a bird. If we find out what this sound is, I'll let you know. Kind regards Vikki