  We present a set of experimental results concerning the power spectrum of
current noise, detected on a granular high Tc superconductor submitted either
to a slowly varying magnetic field or to a varying current intensity.
Experiments were performed on a YBCO specimen suitably treated in order to
weaken the weak links without affecting the oxygen content of grains. The
weakening of the intergrain region allowed the use of very small magnetic
fields and currents to induce the resistive transition of the specimen and to
observe current noise. The induced noise is of the 1/f^2 type and will be
interpreted in terms of two different models. One of the model is based on the
enhancement of the noise due to the clustering of the resistive transition of
the weak links, produced by correlation effects related to the strong
nonlinearity of their Josephson type I-V characteristics. This model has been
the object of a computer simulation based on a 3D-network of Josephson-like
elements and seems suitable to explain the noise produced by current variation.
The second model explains the excess noise as produced by discontinuous
penetration of the magnetic flux inside the intergrain region. This
discontinuity is related to the field screening effect of rings made of several
superconducting weak links connecting different grains, which are alternatively
broken and restored by the current induced during flux variation, and seems
suitable to explain the larger noise produced by a varying magnetic field.
