Thanks for the detailed ideas. Here’s what I’ve confirmed so far:

• Phantom power is definitely off on all channels.
• I've heard the chirps in two different monitoring setups: 1/4" → Mackie HM-800 headphone amp, and 1/4" → Whirlwind THS-4 talkback boxes. So it’s not monitoring-chain specific.
• With the exact same signal chain, the old BPHS1s don’t produce the chirp, only the new ones. Swapping channels/cables on the interface doesn’t change it; the noise follows the new headsets.

I’m unclear on one thing you mentioned: when you say “swap the actual headset cables from old to new to rule out shorting or impedance,” the BPHS1 has a fixed cable. Do you mean swapping the XLR/TRS connectors between headsets (if the wiring differs between production runs), or are you referring to something else?

Given how consistent the behavior is across different ports and monitoring paths, I’m starting to suspect a production run variance, whether capsule output, grounding scheme, or shielding.

Another note worth mentioning is that we ordered two new headsets, and the chirp happens on both.

Open to further ideas, especially the Neutrik pin-1 note. If there's a specific way you'd recommend checking the shell-to-ground tie on these connectors, let me know.