'05 Camry Squeal

Currently driving a 2005 Toyota Camry with disc brakes all around. After driving for about 15-20 min on the highway, I can hear a squeal from one of the rotors (one of the rear ones). I can only hear the noise when the brakes are depressed. As soon as I apply a bit of pressure to the brake pedal, the noise goes away. The squeal isn't constant either; there's a frequency to it, so the noise is most likely related to the rotor.

I can hear the noise at all speeds; the frequency increases as my speed increases. I've recently went to a mechanic but after an inspection (without detaching the caliper housing) he did not notice anything. We went for a test drive and didn't hear anything, but like I said before, the squeal emerges after 15-20 min of driving.

Any comments would be appreciated.