On a number of occasions I had heard a sound that I just couldn't figure out what it was or what was making them, only heard late at night I was sure it was an Owl. With all three common Owls being present in the area it had to be either Barn, Little or Tawny Owl that was making this strange quiet warbling noise but despite researching and trying to track the culprit down I still had no answers. I finally managed to record it out of our bedroom window late one night and heard a Tawny Owl which seemed to be replying? in the background whilst the warbling/trill continued. After searching many websites I found the God's Own Clay website which has a large selection of recordings including the "Xylophone Trill" which finally told me that is was a Tawny Owl. The Tawny Owl calls and vocalizations can be found on their website here
Collins Bird Guides(Mullarney/Svensson/Zettertrom/Grant) describes the following - During courtship either sex utters a low-pitched,shivering tremolo(the so called 'xylophone trill'), o'o'o'o'o'o'o'o'o'o'o'o....., audible only to c.50m.
It's quiet so you'll need to turn the sound up and perhaps your speakers to hear it.
Tawny Owl's "Xylophone Trill".