Earlier this mourning Itai called me for some directions to the Warbler at Ktura . After some 30 minutes he called me back and said the bird is singing the diagnostic Hume's tune with which he is well familiar from
Asia! Some more searching ended with only brief looks at the bird.
I guess my first impression of the bird's call, which I have heard on Friday, was correct, and comparing it by memory to internet records after a few hours was un-effective…
Also around the kibutz's gardens- many Chiffchaffs with 1 Siberian Chiffchaf among them.
Thanks to Itai for solving this short mystery, And allowing me this long expected lifer!