@jetter , @lalitk I tried exactly that - swapping the left and right subs and leaving the connections in place. The problem stayed on the left, so there is definitely nothing wrong with the subs.
When sliding the sub forward or backwards the problem goes away. When I leave the sub in place and move the power and signal cables away from other wiring, then the problem remains. It’s definitely something that happens when the sub is in that exact spot, AND the speakon signal cable is connected, AND there is no signal /music playing. Weirdly, it even happens when the amp is turned off and unplugged. But disconnect the speakon signal cable and no issue when powering on! Very weird problem. But nothing wrong with the subs. I’m going to buy a roll of EMI shielding material to experiment with. It’s not too expensive, and maybe I can try multiple layers if needed. The cover with a small bit of carpet or rug for aesthetics, once the issue is resolved.
@sryeager yes it very much sounded like the end of the world! I couldn’t imagine if this happened when on the other side of the house or downstairs, with this sound blaring! I’d be already of not only the sub damaging but also random stuff in the house breaking!
Even if I fix the issue with shielding, I’m going to be forever paranoid that it will randomly return. As I mentioned, for the first bit, there was no issue.
Update: I wonder if I can get that sound to play when my houses alarm system is tripped. No need for monitoring with that sound playing - it would literally be impossible to be physically in the house with that roar!