  A singularity at the Josephson frequency in the noise spectral density of a
disordered normal metal -- superconductor junction is predicted for bias
voltages below the superconducting gap. The non-stationary Aharonov-Bohm
effect, recently introduced for normal metals, is proposed as a tool for
detecting this singularity. In the presence of a harmonic external field, the
derivative of the noise with respect to the voltage bias reveals jumps when the
applied frequency is commensurate with the Josephson frequency associated with
this bias. The height of these jumps is non-monotonic in the amplitude of the
periodic field. The superconducting flux quantum enters this dependence.
Additional singularities in the frequency dependent noise are predicted above
gap.
