Sounds like the brake rotors are warped. To know for sure your mechanic can measure them on the car with a dial indicator and a micrometer. Since this is so common, most will just skip that step and machine them as long as they'll still be above the published legal minimum thickness when they're done. If they need to be replaced, they aren't very expensive, but new ones often warp in a couple of months and have to be machined too.