What type brakes?

If rim brakes, then sometimes the seam where the rim extrusion is pinned together will be a little out of line and there will be a little bump felt in the brake lever every wheel revolution. More common with new wheels, but a big bump and incorrect spoke tensions might let the joint get out of alignment on a old rim. Not usually a issue. It'll wear in after several rides when the brakes get used enough. Or sand it smooth.

If disc brakes, then check the rotor center lock and/or bolts.

You are sure there isn't play in the headset?