Once while cruising in a 737-200 we got above out target mach to maybe .76 but for sure it was below the barber pole. But all of a sudden we started getting some airframe vibration until the speed was reduced. Is this normal for this to happen when clearly below the barber pole?
Couple of possibilities come to mind.
1. Limit cycle oscillation of a control surface being excited by the higher than customary speed.
2. Unstable shock waves as you apparently are suspecting.
For those who have not had the pleasure of going supersonic, the transition to supersonic in the bird I used to fly was easily perceived by a slight change in pitch attitude nose down and a sudden smoothing out of the aircraft (before it had been like running down a bit of a rough road at high speed). That aircraft did not exhibit noticeable wing drop in the transonic range.
The transition to subsonic from supersonic had an important characteristic. Whatever g we were pulling suddenly increased by ~50% with virtually no aerodynamic warning. Although this created overstress incidents, we did not experience stalls due to the relatively high Q. (This was in a non-FBW aircraft normally operated below FL450.)