Ian's midpipe came back and proved normal when installed on healthy car, conclusion was that he has unusual engine noise (bent valve, failed EGR, etc). We switched him to normal midpipe with fiberglass resonator that is best for absorbing that, but it was a problem with his engine we were working around, not an issue with the midpipe.
Your sound description of back by the rear sounds like heatshield under the trunk, which is a common source on NC. With age the rivets that hold that shield to bottom of trunk get loose and louder exhaust will make it vibrate in sympathy. Solutions include grab it using gloves and tweak it/bend it to again put tension in the way it sits against the rivets. Another choice is remove it and use reflective dynamat or similar on bottom of trunk, removing it is pretty easy by drilling out the rivets.
BUT, it is true as explained in the product description that helmholtz is not working when cold, part of the math is temp and we use normal operating temp for the math. So, if the issue is going away once warm you might be hearing lack of the helmholtz when cold, but then again that same lack of helmholtz when cold could be making that rear heatshield vibrate in sympathy with the 120-130 range the helmholtz chamber trims down as it warms up.