I second the idea that you should replace them regardless. Like I said, when mine got hard, they just squeaked a lot - no loud bang. Also, make sure that there are "pads" above the rubber stops on the hatch. Sometimes these disappear if your car has been repainted and you need these as well to keep the rubber stops from rubbing on the hatch. The pad are essentially felt like material and if you can't find new ones, you can always use some felt and stick it on.

 

I put chair slider felts on the rubber stops, Five years and counting now no issues.They are about 1/8th inch thick (though likely 1/32 now).

But ugly if you notice, I doubt your bang is your stoppers though.

Anything in the spare tire well? Spare tire firmly bolted down?