I modeled the Output Filter Cavity in COMSOL to see if the noise between 3-4 kHz in the open loop transfer function when the cavity is locked is due to mechanical resonances. I modeled the aluminum cavity and its steel supports. See attached photo.

There is no obvious mode between 3 and 4 kHz that would lead to a strong noise source. Modes like the one shown, at 1 kHz, should produce more noise than anything between 3 and 4 kHz.

In summary, this COMSOL model does not support assigning the 3-4 kHz noise to mechanical resonances.