What could explain the fact that during 1976-80, I used to hear commercial jets make sonic booms while flying overhead?
Note that 747's and other jumbo jets operating out of Bradley could not have produced sonic booms because they do not fly above the speed of sound (they only do 500 to 550 MPH at high altitude cruise) and, in any case, far slower than that (~250 MPH) when near the ground as for landing and takeoff.
Therefore, whatever it was you heard, it was assuredly not a sonic boom from a passenger plane. The last time these commonly occurred was in the 1950's and '60s when the US Air Force flew mock bombing missions across the United States in supersonic bombers at low altitude, to train the pilots and crews for similar missions against targets in the USSR.
Although sonic booms were essentially outlawed over populated zones in the early 1960's, in your case they apparently did happen during flight tests of the F-22 Raptor in your location, which is very interesting! Thanks for @reirab's  detective work which uncovered this.