It's more than that, the Flowmaster's have no padding, the mufflers are hollow with baffles.
I had 50 series Flowmasters on my chevelle, right under the floor and it resonated badly at ~ 3000 rpm
Added an H pipe and 2 resonators with no improvement.
Put them on a corvette however, right out the back and it's probably a whole different story.
Car's lookin good Steve, one day you'll see mine, when it's not in pieces, I hope.