Flowmasters are weird like that. I notice that at some point when accelerating hard the exhaust stops getting louder and just gets faster, if that makes sense.
I feel that the reason has to do with either the chambered design of the mufflers (vs bullet style mufflers, like Borla Stingers) or the excessive amount of catalytic converters on our mustangs, assuming you still have the stock mid pipe in place.
Be grateful for those flow masters though. When I got my car it had Magnapacks on it. Talk about quiet and generic sound haha.
Petunia - 05 Mustang 4.0 - Waiting for little brother to come of age
Matilda - 01 Cobra 4V 4.6 - SOLD
Bernadine - 09 Mustang 3V 4.6 - Daily Driver