  Inelastic neutron scattering is used to measure the temperature dependent
phonon dispersion in Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ ($x=0.4$, 0.6). The in-plane
$\Sigma_{4}$ octahedral tilt mode softens significantly at the zone boundary of
the high temperature tetragonal (HTT) \textit{I4}$_{\mathit{1}}$\textit{/acd}
structure as the temperature approaches the transition to a low temperature
orthorhombic (LTO) \textit{Pbca} phase. This behavior is similar to that in
La$_2$CuO$_4$, but a new inelastic feature that is not found in the cuprate is
present. An anomalous phonon mode is observed at energy transfers greater than
the $\Sigma_{4}$ albeit with similar dispersion. This anomalous phonon mode
never softens below $\sim 5$ meV, even for temperatures below the HTT-LTO
transition. This mode is attributed to the presence of intrinsic structural
disorder within the \textit{I4}$_{\mathit{1}}$\textit{/acd} tetragonal
structure of the doped ruthenate.
