my old 1994 9c1 did the same thing and is was a rear axel bearing on the outside eating up the axel shaft. It would make a popping sound espeacialy under light load turning and going over a bump all at the same time. Jack it up and give it a wiggle. 9c1's had a recall on the rear arms due to the intended duty use. They found that under heavy use the rear arms may tear were the swaybar meets the arm. Now keep in mind that all civilian and Impala SS cars have the same setup but were not realled since they were not "to be used as severe duty" cars. Food for thought!