Mystery bird call... Still a mystery!

Hi all, I have been hearing a bird call on and off for years, during the day in Spring, Summer and Autumn, but only infrequently. It isnt a song, just a noise. It seems like it is a larger bird rather than a smaller garden bird. I have attached a recording of the call. Please note, it is the two-tone short hollow noise that repeats at the end of the recording. Its just 10 seconds long - appreciate your help. Many thanks, - G