Just a guess, but I think you may have a rusted out part of your front exhaust. When you are stopped, you are idling at what, 650-700 rpms, but while underway, you are probably well over 2000 rpms and probably 3000 rpms at 75 and pushing much more volume past a hole. Think of it as a wind instrument that requires a certain volume of air to create sound. There's also the possibility that your axles are starting to go, but a roar suggests a hole in your exhaust. Its much easier to see on a lift. Next time your car is on a lift, ask them to check your front pipe.
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