It really should not be the lack of flex pipes causing the drone, it has to do with standing wavelengths through long pipes. The length does matter a lot, what you need to know is the frequency of the drone. It should be close to what mine was.
Is the drone on your car heaviest between 2200rpms and 2800rpms? If so, you need around a 28" side branch resonator on each side, coming at a 90* angle off the area between the tips and the rear mufflers. That will kill off most of the drone. I had a local shop with a good welder make them for me for $100 and then I coated them in 2000* high temp flat black paint. They are tucked up behind the muffler so you cannot see them. Make sure to cover your rear bumper and the car undersides before you paint anything so you dont get spray paint on your paint job!
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2013 Ford Mustang GT
374rwhp, 360rwtq SAE / GT500 Axle-Backs / Brembo Pack / Track Pack / Recaros / 6spd
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