Sony UWP-D Audio Issues
After searching forums and YouTube videos, I believe I've found the culprit:

My building's ultrasonic motion detectors. 

They emit an ultrasonic chirp every few seconds that the mics 'hear' and turn the volume down (attenuate?) for that portion. 

I've contacted Sony Professional Equipment support and I've been issued a Beta Firmware upgrade for the transmitters (that's where the issue is, apparently) that's worked on one of my UTX-BO3's and my UTX-PO3. 

The other BO3 got the firmware but has developed an issue where it can't stay on for more than 2 seconds on battery power. So it looks like I'll have to send it in. I'll update once I hear more.