My Valiant kind of goes against the grain for some of the experiences & theories out there. It's a mild 340 (300hp), with 1-5/8"x1-3/4" TTI step headers in to 3" H-pipe & Super Turbo exhaust. I have no carpet on the interior of the car (leaks), and I have no drone at all. Same system on a stone-stock 74 318, no drone. The previous system was a 2" dual with cheapo turbo muffs and no H-pipe, and it had a light drone at certain speeds with the 318. Never tried it with the 340. The TTI 3" system is actually quieter on the road and deeper at idle than the cheapo system with both engines. I think it's louder at WOT, though.

The passive radiator situation could be the culprit, as my carport buzzes and hums loudly when my Valiant sits at a warm idle under it. Really strange sounding. Something like that could be going on with your car as well. I'm guessing that the window bumpers and trunk seals are in good shape and functioning?

Another issue I haven't seen mentioned is driveshaft or u-joint harmonics. I got a bad drone when I replaced my axle and the driveshaft angle wasn't just right. Adding adjustable hangers in front made that go away. Maybe try rotating your driveshaft 180* on the rear yoke and see if that does anything.

Clair