Good to hear you didn't have to tear into anything expensive only to find nothing wrong.
Broken radial belts are more common than is realised. A sidewall bounce off a curb or a crash through a deep pot hole is sometimes all it takes. It can even start simply from underinflation which builds heat in the tire from excessive flexing.
Often they start out as a very slight "waddle" when creeping along in heavy traffic under 10 or 15 mph. As they get worse it develops into a thump and vibration similar to an unbalanced tire.
