What's my noise?

Last night (in the early hours) my wife and I heard a noise from outside (in a residential area) - it was a low, clearly animal, moaning and went on for a few minutes then abruptly stopped. It was most definitely not cats (they have more of a baby wail) but was certainly something in distress / pain. We have rabbits (both are fine, I went to check on them straight away) and our neighbours have some hens but I would expect that if they were attacked it would be more of a birdy squawking? It was a really disturbing noise and it's really un-nerved my wife – we just can't put our finger on what it could be. Ideas anyone?