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Me thinks you need to damp that enclosure. It could be done from the inside or the outside. Consider the asphalt based damping sheets available from Parts Express on the inside and/or indoor/outdoor carpeting on the outside. I've built a Shiva-based Sonosub and an AV8 based tube, a little different from your design, yet I've never experienced ringing. I have experienced resonance in a ported Sonotube design. Perhaps the stiffness of the PVC contributes to that, being a material that may get into sympathetic vibration with some of the higher harmonics.
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