  We observe the suppression of the finite frequency shot-noise produced by a
voltage biased tunnel junction due to its interaction with a single
electromagnetic mode of high impedance. The tunnel junction is embedded in a
quarter wavelength resonator containing a dense SQUID array providing it with a
characteristic impedance in the kOhms range and a resonant frequency tunable in
the 4-6 GHz range. Such high impedance gives rise to a sizeable Coulomb
blockade on the tunnel junction (roughly 30% reduction in the differential
conductance) and allows an efficient measurement of the spectral density of the
current fluctuations at the resonator frequency. The observed blockade of
shot-noise is found in agreement with an extension of the dynamical Coulomb
blockade theory.
