Ed,
I have just come to the exact same conclusion. Now that I've done some listening, I realize EVEN the stock car for example has higher frequency humm, very small in volume but there & most likely a resonance, at around 2500 to 3000 when throttle is applied in a certain way, with car at some combination of speed/gear.
In after market exhausts this noise is louder. I guess how problematic it is depends on how loud it gets, if it becomes a physical buzzing, & how persistent it is. In talking to owners with after-market exhaust and in direct listening, I have heard the following comment more than once: "There is a noise here that will go away if you select a different gear." I am still learning, but suspect that all 3 systems I am considering, AWE, Cargraphic, and Tubi, have some component of this.
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edz61 said:
Trust me on this and I don't care who says what all hi performance exhausts have a thing I call annoying point at a certain RPM.