I used the buzzer to help locate sources of scatter in the transmission beat board. 

Here I used the SR785 in swept sine mode: 10kHz to 20 Hz, 10 mV source, a 35 mV offset, amplified by a thorlabs HV amplifier (x15). I took the mechanical transfer function from the buzzer to the PLL actuation signal (the same used to measure the displacement spectrum).

Below I've noted the frequencies and the amplitude in units of dB (ratio of PLL signal to excitation 10 mV). The frequencies listed in the upper half of the labels are buzz applied horizontally on the mount, those on the lower half are resonances excited with vibrations coupled in the vertical direction.  I would have made this labeling clearer, but I saved accidentally closed as jpeg so its been flattened.

Vertically applied excitations seem to couple down into the board more easily and common frequencies may well be same the same scatter source being excite from different points in the board. There are some standard resonances from vertical excitations at 1.03 kHz, 1.13-1.15 kHz, 3.11-3.2 kHz. These can also be seen by exciting the elevated breadboard directly. Maybe we can apply some sorbothane dampeners to the underside of the board to kill these resonances. 

Horizontal resonances were less likely to be common between mounts.  

Note that since I took this picture the ND filters were removed from directly before the PD.