[ayeckley]

We also found (years ago) that they are sensitive to "environmental" ultrasonic noise. Such as that produced when two hard objects (like gear teeth) smack together and produce a broad spectrum of frequencies. Like on most robots...

No, we don't have spectral measurements to support the theory, just a basic knowledge of sound and vibration physics. It masks as an EMI issue, but when the gear teeth (and only the gear teeth) were removed from the test scope the problem effectively went away. YMMV.