Hey guys, I've got an 03 4.6is. Everything is stock except for a hartge rear section. Sitting at idle, i get a low rumble and a bit of vibration when sitting still in P or D. It only started doing this a few months ago, but has recently gotten worse. Has anyone experienced anything like this?

Im still under CPO, so Im going to let the stealer handle this headache...but I just need to know what to tell the SA if someone has seen this before.