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Get the same from my H&S original SS pipe too - when its heated up it sound Fantastic
I believe it is because the mechanical properties of the stainless steel change as it gets hotter and thus the resonant frequency of the exhaust changes, giving the exhaust a deeper sounding note. This can only be engineered out of the exhaust by modelling it in a finite element analysis program on the computer to find it's natural frequency and where the most displacement is when it is subject to the exicitation forces from the engine.
I hope this helps (not sure if the scooby engine changes note slightly either as it gets warmer)