It seems to happen anytime after the car's been sitting for a few hours. I particularly notice it every morning as I pull out of the parking lot, and it definitely started when they replaced the shocks last week. It's really loud, as loud as someone throwing a rock at the trunk.

I was already hesitant to replace the shocks when I wasn't noticing any particular problems with the back end. Now I'm pissed that it created a whole new problem that didn't exist prior.