The driveline knocking came back. I tried replicating it with the wheels in the air, car in gear, forwards and reverse, no joy. I inspected the drive shaft u-joints, no joy. I looked at the pinion play and halfshaft play, no joy. I found a lot of threads with similar issues, but rarely any resolution.
I made a video of the sound, from in my trunk, just moving forward and reverse normally in my driveway. You can hear the knocks.
I searched the internet using various phrases and combos and stumbled upon one at another Miata forum. It referenced worn UCA (upper control arm bushings) with pics of how that person's bushings looked when the noise occurred. I ran out side, peaked at my UCAs. Bingo! Here's what mine looked like:
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RR UCA
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LR UCA
See how the arms are shoved to one side, plat against their shackles on the rear sub frame (RSF)? Yeah, that's not supposed to be like that. What I believe is happening is the UCA is banging against the RSF under accel and lift. This sort of explains why the noise was moving around on me, first it was on the right, then the left. I ordered the Energy Suspension rear bushing kit. I will tackle the UCAs, so as not to disturb the alignment, then we will see how this goes. I was worried my diff was on the way out, I had already put in notice I wouldn't be at Road Atlanta next week, but this may save the event for me! I hope the bushings arrive when they say they are!