What sounds like a corncrake?

A relative of mine was out walking this morning around a farm and she phoned me to say she had heard something that sounded like a corncrake.  As it is not the season for corncrakes to be appearing I'm suspecting it is something else but I can't really think what.  Both of us walk around the farm regularly and are pretty much used to the usual bird calls that go on there.

Anyone have any suggestions for other birds that could be in the Aberdeenshire/Angus area of Scotland?