Pipeline vibrations can be caused by hydrodynamic factors and vortex formation at local resistances, as well as induced vibrations from connected equipment. In some cases, the situation is complicated by resonance with the natural frequencies of the pipeline system or by beating when forced frequencies coincide. Under such conditions, pipeline vibrations may exceed permissible norms by 1.5–2 times. This study focuses on determining the frequencies of hydrodynamic-origin vibrations in tees operating with lateral branch flow. The frequency range of hydrodynamic vibrations directly depends on the flow velocity of the working medium. As the velocity decreases, the frequency range also narrows. In tees, vortex formation occurs at sufficiently high fluid flow velocities, leading to forced vibrations. Numerical values of the Strouhal number, which characterizes hydrodynamic phenomena in unsteady flow regimes, obtained as functions of the Reynolds number, can aid in predicting characteristic vibration frequencies in complex pipeline systems.