Annoying clanking noise from the back of the car

The animated GIF file above is muted and the volume needs to be adjusted to hear it. The gif file is also muted by default at the Imgur site.
The sound is almost the same to my ear as the door lock. Other guesses are some actuator or valve trying to switch on, off or open, close.
My dad was a mechanical engineer and unknown rattles, thumps, clunks, etc used to drive him bonkers too.

 

Fourth gens had service bulletin for a loose rear suspension bolt (or nut) IIRC?

 

Did you figure this out? I had a rattling sound I couldn't figure out coming from inside the car when going over bumps. One day my wife noticed that the cargo cover was upside down. I removed it a while back to fit a bunch of stuff going to Goodwill. I must have put it back in wrong without paying much attention. I almost never use it, so I never noticed it was on upside down. Once I fixed that, the rattling went away.

 

I heard what I think is the same sound in my Prime this morning, as the sounds I heard in the GIF in the OP.
I had driven about 10 mile and was standing in a parking lot in READY mode and switched from HV mode to EV mode while looking at car scanners All Sensors. It was 30 F ambient and coolent temp was 158 F . I switched back to HV mode and turned on the heat to 68 F I wanted to time how long it took for the coolant to drop below the temp where the engine would start. I heard the sounds 2 or 3 times from the back of the car, sounding like the door locking sounds, but further back and underneath. My first thought was something switching inside the traction battery case, from the perceived location and barely noticeable even while the car was parked in READY mode without the engine running. I'll post here, if I hear it again and have any clues what it might be.