89’ F250 7.3 with turbo. Straight 3” exhaust, in good shape (new). The exhaust is not loud at all but I have an annoying resonance in the cab. I have dynomat on the floor and roof of the cab, and the truck is pretty quiet for an idi. between about 65-75mph I get a loud drone in the cab. This is right where I would like to cruise on the highway of course. It is definitely a resonance thing because if I move my head forward over the steering wheel, the noise disappears. Also if I get up close to 80mph it disappears. Anyone have experience with adding a resonator?