What is causing this rubbing sound from my brakes?

I’ve now taken this to no fewer than three bike shops, and the problem seems to keep coming back. So I’m thinking I need to take matters into my own hands and figure this out despite knowing very little about bikes.

Every time I ride downhill – and my house is on quite a steep hill so that’s often – it comes back for a while exactly as you can hear in the video. It’s an annoying sound, and I can sort of feel it too.

I have checked the brake pads – definitely not fully worn down, and tried to realign the brake itself with the rotor by loosening the mounting screws, squeezing the brake and retightening them. Still hasn’t fixed it.

From what I can see, the rotor isn’t bent. I don’t see any side to side movement when I look at it while riding.

Would love your help! 🙏