The blankets in the trunk is a worthwhile experiment.
So is silicone caulking cracks that might vibrate, or filling voids made of flimsy sheet metal.
Another worthwhile experiment is to use Vice-Grips to clamp different sized sockets inside the exhaust pipes at the outlet. This slightly increases the back-pressure and changes the speed of sound in the exhaust gas.
You already mentioned you can hear a difference in the drone was the engine coolant temp went up. That is also a speed of sound in hotter gas effect.
The pictures you have posted seem to show you have room to add closed end T- stubs. These are typically 26 to 18 inches long. Think of these as “blind alleys” the sound wave charges down, hits its head on the closed end, and dies. Start long and trim off length until the sound cancellation is greatest at your cruise RPM. The stub tubes can be bent, even coiled. They can also be the next smaller diameter pipe with just a small decrease in effectiveness.
You could also extend the length of your exhaust pipe past the critical 70 inch length somehow., like have the driver’s side exhaust exit on the passenger side, and vice versa.
I just stuffed a blanket, a comforter, a pillow, a large sleeping bag and two foldable mattresses in the trunk and it didn't do much of anything. Granted, there was still a void between the battery and the quarter panel and a little over the rear end but it was 90% filled. I just didn't want to mess up the sheets or anything. I tried to film myself driving the car to capture the drone and ironically, my phone cannot pick up the drone frequency. So, everything sounds fine on the video when it is obviously annoying in real life.
I can call the exhaust guy to see if he's heard of or would be interested in a Helmoltz-style resonator, but it didn't seem like he had any idea of what it was or he probably would have brought it up.
Is that 70" past the muffler? Rear End? Does it have to be a straight line of tubing?
