If you look at our beatnote ASD you can see a broad dirty hump at 500 Hz.
awade played a pure 500 Hz tone through our lab speakers, and you could see the resonance peak being driven.  Blue are the driven spectra, orange are the non-driven spectra.
If we turn up the speakers it causes our North path to lose lock.

Some sort of mechanical resonance in the North path is causing this.  Buzzing is underway.