  We investigate two distinct universality classes for probe particles that
move stochastically in a one-dimensional driven system. If the random force
that drives the probe particles is fully generated by the current fluctuations
of the driven fluid, such as when the probe particles are embedded in a ring,
they inherit the dynamical exponent of the fluid, which generically is z=3/2.
On the other hand, if the random force has a part that is temporally
uncorrelated, the resulting motion can be described by a dynamical exponent z=2
as considered in previous work.
