BDR 1340 does the same thing. and I have taken the rubber mallet to them as well. It is not the 585 hp, because I have the 5.0 Coyote. It looks to me like back pressure, when you watch the pipe with the door open. All I know is its annoying. I talked with Jay on changing the mufflers out. If that noise is normal I don't want normal
Jay and Brian will figure it out you can be sure of that.