If the trailer hitch wiring didnt do anything, its more than likely a problem caused by the LED tail/brake lights. either switch them out for incandescent or put load resistors around them like you would on turn signals
I know on mine when it was happening, if i had the car in park and hit the turn signal, i could hear the shift lock releasing in the center console too (this could save you trying to drive and test with cruise control. if yours make the same sound, just see if the sound goes away with each thing you change)
if you do a search for "turn signal cruise control" you will find my old thread on this lighting issue and what i did to locate the problem/solve it