  In this paper, we present experimental results and reveal that strong
perturbations are not necessary for elastic instability to occur in
straight-channel, inertialess, visco-elastic flows at high elasticity. We show
that a non-normal mode bifurcation is followed by chaotic fluctuations,
self-organized as stream-wise streaks, and elastic waves due to weak
disturbances generated by a small cavity at the center of the top channel wall.
The chaotic flow persists in the transition, elastic turbulence, and drag
reduction regimes, in agreement with previous observations for the case of
strong perturbations at the inlet. Furthermore, the elastic waves we observe
propagate in the span-wise direction, which allows to confirm the elastic waves
linear dispersion relation directly for the first time. In addition, the
span-wise propagating elastic wave's velocity depends on $\mathrm{Wi}$ with the
same scaling that was previously observed for stream-wise propagating waves,
although their velocity magnitude is significantly smaller than what was
previously observed for the stream-wise ones.
