Nocturnal bird sound?

[hel_is_heaven]

Hi,
I have a conundrum for those of you in the know about birds.
Last night I heard a sound that I have never heard before. I don't have a sound clip of it, which I know makes this so much more difficult.
To be honest I am not even sure it was a bird, the only reason I think it might be is that is went from left to right and back again pretty fast outside my window.
The noise was an elogated Awaaaaaaa Awaaaaaaa. This was about 3am in the morning I heard it, so I presumed it was a nocturnal bird but of course I could be assuming that wrongly.
I live in London suburbs if this helps, however I am thinking that as I haven't heard it before that it may not be something that is normally in London?
I would appreciate any sound clips that you think could be what I am describing, so I can listen to them and confirm if its that or not.
Thanks for any help you can give me!
Helen

[shenk1]

Try green woodpecker. We heard a similar described sound last week just before dawn and saw one under the street light collecting ants from a gap in the pavement.

[loripo]

Have a listen to this on RSPB
The RSPB: Green woodpecker

see if that helps.

[RoyW]

Sounds to me that it was probably a gull, most likely a Herring or Lesser Black-backed Gull. Both make a variety of similar sounds and are increasingly breeding on buildings around London (+ breeding birds from elsewhere are starting to return). These, and smaller gulls, are often out and about before it is light scavenging fast food and other scraps from pavements before people wake up.

The RSPB: Lesser black-backed gull

The RSPB: Herring gull

Roy.