What an enigmatic song! It seems like a puzzle, and the truth is that I don't know what else to say. I've been looking at the sonogram and comparing it to different species that may sound similar (even with some rarities), and it doesn't fit well with any of them. It seems pretty sure to me that it is not a Western Olivaceus Warbler, but sometimes I find the recording to be good for the Blackcap subsong, other times it seems better for the Garden Warbler subsong. On the other hand, either there are imitations or there were other birds in the background, because sometimes the typical "chilp" of the House Sparrow is heard, and at other times typical Nightingale sonograms are seen. So, I have no idea what it could be, let's see if someone manages to solve the mystery!