When I touch my brake pedal lightly I hear this loud humming sound which vibrates the car a little. When brakes are pressed harder it goes away. I think it is the front brakes because I can pull the emergency brake a little and it won't do it. It will make the noise then stop then make again like it is just one spot on the disc. What could this be?

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89 IROC L98 3.27 BW
Hypertech stage 2 chip, Airfoil, 3 inch exhaust, gutted cat and MAF screens