Very unusual bird song - help needed with identification please.

Heard at dusk this evening, 28 Nov 2020, in a small area of birch woodland. Two birds of about rook-size, but definitely not rooks. They were a longer and slimmer shape. It was too dark to see any colour, they appeared black against the sky.

The song consisted of 2 or 3 elongated and quite musical notes "wow" or "woo" followed by a sound like very rapid beak clacking. This was repeated several times, I think by both birds. Sadly I wasn't quick enough to get a recording.

I've looked on RSPB and other websites and can't find a song like it. Would be very grateful for any suggestions as to what this might be.