A sound

Observation - A sound. Description: Sorry, no picture. On Dartmoor this weekend we heard a call sometime around midnight. It was a loud 'round' hollow sound rising slightly at the end, sort of oooooo-ip about 2 or 3 seconds long repeated a few (

Sorry, no picture. On Dartmoor this weekend we heard a call sometime around midnight. It was a loud 'round' hollow sound rising slightly at the end, sort of oooooo-ip about 2 or 3 seconds long repeated a few (<10) times - a bit like a gibbon. Might have been bird but sounded more mammal like. Fox? Deer? Any ideas?