It turns out my big blue party fell and afterwards the sub was buzzing on heavy base. I used a signal generator app on my smart phone to isolate the noise. In debugging it I also found that removing the sub or the passive radiator eliminated the noise. I tried a replacement sub and the noise issue was still present. So I looked for a physical vibration inside the speaker such as cables, wires,circuit boards or the enclosure itself. After many tries to shore up potential noise sources I realized that there was acoustic energy escaping from outside the driver mounting interface along the edge. I made a gasket out of thin rubber foam and applied it to the mounting surface and then remounted the sub with its 4 screws. Voila, the buzz was gone. The enclosure was now sealed. For some odd reason the impact the enclosure made with the floor seemed to permanently affect the seal the sub makes with the enclosure. This whistling effect is only revealed with high energy low frequency signals.