  We report measurements of voltage noise due to vortex motion in Niobium, a
conventional low-Tc superconductor. A coherent oscillation leading to narrow
band noise (NBN) is evidenced. Its characteristic frequency is a linear
function of the overcritical transport current in the flux-flow regime, and
hence scales as the main velocity of the vortex flow. The associated length
scale is not the intervortex distance but the width of the sample, indicating
temporal coherence at a large scale. NBN is also observed in the non linear
part of the V(I) at the onset of depinning, in apparent disagreement with a
stochastic creep motion of flux bundles. NBN exists in the peak effect region,
showing that long range temporal correlations are preserved in this regime.
