re: Need to hear your positive or not experience with a good auto exhaust shop in BR

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I believe it's the exhaust also because I have been able to change it by putting clamps on the exhaust pipe at various locations and it changed the frequency of the resonance.

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Like just external clamps? Were they tight and supporting, so it would eliminate vibration?

When you say harmonics, it could be flow induced or vibration induced. If vibration, you should be able to clamp it/support it and it would eliminate the noise, which sounds like what you've done here.

If it flow induced, you would have to change the length of the run, or put a restriction in the pipe. This can't be done with just a clamp.

Also, as mentioned, I had a difficult to locate resonance that was the result of a leak at the "donut gasket" in my old car. Only really noticeable under hard throttle or deceleration.

If clamping changes the tone, you can try installing new hangers or tying the dual pipes together (if two straight pipes). If flow induced, repositioning the cats or muffler, installing an X/H pipe (if not installed), or changing the pipe size could all help.