I was experiencing odd suspension behavior which developed gradually, but when thumping and bumping sounds started from the rear, I took a look underneath, and saw that the left rear shock had leaked - or sprayed - out its fluid. Now its awaiting a replacement shock at the service shop. I've read that these cars were built with failure-prone shocks. If you are experiencing odd ride sensations and/or hearing clunky sounds from underneath, take a look for griminess around the shock or strut. That's where road dirt stuck to leaked fluid. If the shock or strut on the opposite is much cleaner, you've likely got a blown shock.
Former driver of a 2011 SLS AWD Platinum, currently driving 2015 Mazda6