You may want to try installing Moroso spiral flow race mufflers.

I had headers and straight pipes (no cats) on the sig Viper, LOUD and NASTY. Added a pair of the Moroso spiral flows and the nasty edge went away with a lower tone.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!