Flowmaster drone solved.

I had exhaust drone issues on my 84 Mustang back in the day. 289, c4 and 3.27:1 gears. Was running 40 series Flowmasters. Swapped to 50 series deltaflow Flowmasters and no drone. Was running full 2.5" exhaust, H pipe and tips. Every vehicle is different. Current GT has 40 series and no drone.