On several occasions whilst walking in Brecon Beacon valleys, this summer, I have heard a call that sounds very similar to a grasshopper or cricket's "chirrup", but at a volume that suggests something much bigger. I have never actually seen the caller. More recently, whilst near a sports ground in Bedfordshire, a similar sound was clearly coming from the lower branches of a tree. The caller seemed to move to other branches as I circled the tree trying to locate it, and I did fleetingly see a bird between Sparrow and Blackbird size fly from one tree to another, closely followed by the sound coming from the new tree.
Perhaps not much to go on, but anyone have any thoughts on what the caller might be?