Front brakes vibrate when stopping

Used rotors must be machined to true them to match the new pads. When they are not machined, they will have uneven surfaces that matched the wear on the old pads so they made 100 percent contact. With that uneven wear, the new pads contact a very small area of the rotors, and those sections do all the stopping so they get unusually hot. That can lead to warping the rotors.

There are a lot of things professionals do as part of a normal brake job to prevent noises and pedal pulsations. High-temperature brake grease is used on a half dozen key locations, and the pads are prepared to prevent squeals. Simply hanging new pads on the car doesn't address all of the things that can cause problems in the future.