Rear suspension clunk on deep bumps

I have a 2015 Impala 2LT with 3.6L engine and 54,000 miles, purchased Certified-Used with 49,000 miles less than a year ago.

I have a clunking sound in the rear when going over medium sized bumps. On smaller bumps and dips, the suspension handles it beautifully. I took it to the dealer and they could not duplicate it and found that rear shocks, stabilizer bars, etc are fine.

Any idea where else to look for the source of the issue. There are no loose or rattling trunk items causing the noise. Front suspension seems fine.

Thanks for any ideas you might have.

Mark