does anyone have a clue what this might be? the noise comes from underneath, a constant metallic ringing that sounds like a cycle (also sounds to me like a bunch of cricket echoes in the woods!) it amplifies to a high pitched squeal when the car is in reverse. the only time it stops is when there’s pressure on the brakes, or the wheel is significantly turned.
Your brake pads have worn down to the wear sensors which are little metal pieces designed to scrape on the rotor, making a squealing noise. Have your pads replaced soon or you may risk damaging the rotor surface which could require replacement of the rotors as well. You just need a brake job.
N55 is 100% correct. You need to have your brake pads (check both front and rear pads!) replaced a.s.ap. Just in case the noise suddenly stops, that just means that the wear indicators are now worn out, and that you will soon be braking with metal-on-metal, rather then with brake pads on metal. (Hint: That is not a good thing.)
Take the car in now for a brake job before you multiply the cost of the repair.