  The shot noise of current through a metallic double quantum dot structure
exhibiting negative differential conductance is studied. We can exactly solve
the master equation and derive an analytical expression of the spectral density
of current fluctuations as a function of frequency in the first Coulomb
staircase region. For a large range of bias voltage the noise is calculated by
Monte Carlo simulation. We show that the noise is always sub-Poissonian though
it is considerably enhanced in the negative differential conductance regime.
