We have a 28 foot travel trailer with tandem axles. Yesterday while backing it into our driveway, my son noticed a steady, high-pitched noise coming from behind the right rear wheel any time the trailer brakes were applied. Most of the time it was while the trailer was sitting still, but also while rolling either direction. The sound is clearly coming from the brake system behind the wheel. The brakes seem to be working exactly as normal, and they release as well, and the noise is only from the one wheel. The noise always stops the instant the brakes are released. As I understand, the brake is actuated by an electromagnet. What would be making the noise in that mechanism?