The treatment depends on if the drone you hear is structural or airborne noise related. If structural, adding damping at the attach points of the exhaust would help minimize the energy getting into the structure. If airborne then the mass addition to the interior panels is the way to go only after identifying which panels are the greatest contributors. If most of the airborne noise is vibrating your windows you are pretty much screwed unless you can change the modal characteristics of the window ("add a heavy tint layer"......joking).
Notice all the "ifs". Careful effort identifying the "source-path-reciever" will enable the least amount of weight addition.
You gained HP why throw some of that goodness away by adding unnecessary weight?
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Buddy Revell: "One man's sooner is another man's later"
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