Coming back to this thread after another three years. It has been doing it on and off for the entire time, I learned how to live with it by switching modes as cmsee17 had been doing.
Living in NYC on my setup the interior has heard* better days. Still looks good. Harsh bumps can trigger it from my experience. This is leading me to think there is a hardware/grounding issue with the combox or related board. I will be tearing apart the trunk at some point to figure it out. Worst case is one or both of these.
5/2012 non-nav with HK sound
Harmon Kardon Head Unit Radio Cd Unit Oem
https://www.ebay.com/itm/17534834901...pid=9020507460
Harman Becker Telematics Bluetooth Control Module
https://www.ebay.com/itm/40291587939...Cclp%3A2047675