Noise from back passenger side
I hear a persistent wub wub sound when I am braking, seems to come from the back passenger side of car. Mechanic says brakes are fine, rotors are fine, but it's concerning. Of course when I had him in the car with me to try to pinpoint the issue, it wasn't happening. I would describe the sound occurrence as consistently random, if that makes sense. Any idea of what might be causing the noise?
Thanks to all and any who may assist.