I'm guessing drones. This was from an article in September, before the recent conflict, talking about the constant noise of drones flying overhead is an ever present source of fear and anxiety in Gaza.
Some excerpts:
"Unmanned surveillance aircraft have become an integral part of Israel's 15-year-old blockade of the impoverished enclave, and 2.3 million Palestinians endure their incessant hum."
"Gaza teenager Bissam says she has trouble sleeping and concentrating as the buzzing sound of Israeli military drones above the crowded Palestinian enclave drives her to distraction."
"Sometimes I have to put the pillow on my head so I don't hear its buzz," she said, adding that the drone noise gives her headaches."
"He told AFP that "the buzzing of the drones and the intermittent raids of the F16 (warplanes) have become an integral part of our lives"