According to Matt's alog 26499, the 1/f^2 mystery noise needs to have an amplitude of about 1e-20m/rtHz at 100Hz.

With the ESD actuation function ( ~6.38e-8m/N * (100Hz/f)^2 ) this corresponds to a flat force noise of about Fc = 1.5e-13N/rtHz.

We can cast this into a voltage noise at the ESD F=alpha*V~2:
Parameters:
alpha-~2e-10N/V^2,
bias voltage V0-380V.

For the linear coupling this implies V = Fc / (2*alpha*V0) = 1.0 microV/rtHz.

In principle the quadratic nature of the ESD can also down-convert higher frequencies. For a band of white-ish noise with a bandwidth BW, the resulting force noise is F_DC = alpha * V_RF^2 * sqrt(BW). To produce the mystery noise through this down-conversion, assuming a noise bandwidth of BW~1MHz, would require ~1mV/rtHz at RF frequencies.