Today, while programming the nav and car was in acc mode, I heard a very audible "tap" coming from the rear of the car. I got out and listened all around until I located where the sound seemed to be coming from. The "tap" seemed to be coming from the driver side rear wheel/suspension area. It occured only while in acc mode and went away as soon as I turned off or cranked the car. It "taps" at a 2-4 second pace and there is no vibration to be found from where the sound appears to be coming from, i.e., it's not something hitting something, it's more mechanical. Anyone know what this could be?