When my car is completely warmed up and I am sitting at idle, I hear a weird popping sound from the rear passenger side. I can actually also "feel" when it pops if my hand is on the handbrake or shifter. It could be described as a knocking, too. Almost like something is faintly knocking on the underside of the car. I looked under the car while it was running and there was no popping heard from the outside, but if I went inside the interior I could hear the popping. It is very strange. Has anyone had a similar issue? I thought it may have been the exhaust moving around but that doesn't seem like what it is. This only happens at idle. The car is completely smooth when driving with no popping sounds.