Vibration/thumping when braking
 

I have a 2012 Audi A4 Avant with 73,000 miles. At 50,000 miles I had the brake rotors turned and the calipers adjusted. Lately I've noticed when driving downhill, but only on some downhill sections, on I70 heading down from the mountains towards Denver, a vibration/thumping when braking. The thing is it only happens on certain downhill sections, not all. I know the prudent thing is to get the car checked. But might it be just the road surface in those two areas? Thanks.