HFC typically make a noticeable difference in sound and can cause drone and rasp. Adding a CAI shouldn't make it any worse. Typically CAI will make more of an engine growl under hard acceleration, but there won't be any difference under normal acceleration. Adding a CAI will then allow you to further mod the air intake by upgrading or porting your throttle bodies and the upper and lower manifolds.
Although a gear and diff swap will make a noticeable difference, they aren't easy to do and can be pretty expensive, especially if you have a mechanic do the work.
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~Rob 
-DD Air Guide, Stillen G3 CAIs, 63mm Throttle Bodies, Motordyne M370, Berk HFCs, FI TDX CBE (non-res, CF), Sprint booster v3, Ecutek Dyno Tuned, and the list goes on...
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