Haunted by mystery bird call and slowly going crazy! Please help!

Every day, at several intervals, I hear a very distinct, unmistakable two-note call.  It's relatively low-pitched compared to other birds I hear nearby, and comes out of nowhere - it sounds once, and is then silent for long periods of time.  Every time I hear it I rush to the window but can't see where it came from!

It's particularly noticeable because it usually happens when things have quietened down outside in the evening (it's summer now).  I can't predict or record it because it always happens just once, all of a sudden.  Two notes then nothing for hours. The two notes are almost the same but I think the second one is slightly lower.  It's not a chirp, it's more of a whistle.  And it's always the same.

There is a hedgerow outside my house, with woods down the lane, as well as fields.  I see and hear many different  birds, but cannot identify this one for the life of me.  There are house martins and starlings in my eaves, and I often see sparrows and wagtails too.  I don't think it is them?

It's been bothering me for weeks!  Any ideas what it could be?