I have a 2012 CTS4 Luxury that has recently started to "rumble/vibrate" under slight throttle load at around 60 mph. It lasts for a couple of seconds and seems to go away. It feels like a possible transmission issue. The car has 112K miles on it, much of it highway. I just got a tune-up (new plugs, etc), but that didn't seem to help. I haven't taken it to a dealer for diagnosis, yet. Is the transmission about to go out? Any ideas? Thanks!