  Using numerical simulations we examine the velocity fluctuations of a probe
particle driven with constant force through a two-dimensional disordered
assembly of disks which has a well-defined jamming point J at a density of
\phi_J=0.843. As \phi increases toward \phi_J, the average velocity of the
probe particle decreases and the velocity fluctuations show an increasingly
intermittent or avalanchelike behavior. When the system is within a few percent
of the jamming density, the velocity distributions are exponential, while when
the system is less than a percent away from jamming, the velocity distributions
have a non-exponential or power law character. The velocity power spectra
exhibit a crossover from a Lorentzian form to a 1/f shape near jamming. We
extract a correlation exponent \nu which is in good agreement with recent shear
simulations. For \phi > \phi_J, there is a critical threshold force F_c that
must be applied for the probe particle to move through the sample which
increases with increasing \phi. The onset of the probe motion above \phi_J
occurs via a local yielding of the particles around the probe particle which we
term a local shear banding effect.
