I recently purchased a used 06 Charger with the 5.7 HEMI. During acceleration there was an annoying growl or vibration coming from the exhaust. I had the exhaust flex fittings added thinking it may help. Nope! I had the catalyct converters tested. Ok. I added friction modifier to my transmission thinking this may be the torque converter noises I've seen in other forums. Nope! (However did make it shift smoother) So I took it back to a different muffler shop and asked them to find the annoying rattle. Turns out the last person who changed the transmission fluid out installed the transmission mount cross member upside down. This was vibration on the transmission during acceleration. Flipped it back over and the car sails down the road, quiet as ever....