had car weighed down with 5 boxes of tiles and heard a loud snap kind of noise

Either your shocks or rear springs caused the snap. Does it sit or ride different? Some times with no weight and you go over a rough spot, R X R tracks or pot holes a spring may pop out of the cup it sits in and that extra weight may have snapped it back into the cup. Check the shocks and springs for any visible damage, broken spring or a leak in the shocks.

For some reason this site seems to double post answers so I deleted my dual second answer.

Same scenario on our 2008HHR. The pass rear coil spring was broke on the bottom coil, the car never leaned at all, so a close look will be needed.

These are the type of questions that when a fix is found, it would be very helpful to post the solution.

Ok, I should of included what I did for the repair. I did replace both rear coil springs, purchased from Amazon for less than 40.00. Had to support the main car body, put a floor jack under the middle of the axle, raised the jack until contact was made, remove the bolts at the bottom end of the shocks, removed bolt holding the rear brake line bracket to the car, hand loosen bracket so when the rear axle is slowly lowered, the metal brake line will not kink. Lower axle just enough to slightly pry springs out of the containment. Seems as if I had more room to work with the rear tires off. good luck.