I have a 68 charger that had serious drone when it had stock manifolds. When I installed headers the drone was gone. I'm running summit turbo mufflers with a 2.5" exhaust.

I also had a 78 Trans Am with a 2.5" pypes exhaust with x pipe with dyno max mufflers. It had serious drone from 42mph to 50mph. It had stock exhaust manifolds. Part of the problem is that trans ams have exhaust tips that aim at the ground which amplifies the drone.