In-core vibration measurements are crucial in view of preventing excessive vibration already in the design stage or assessing component integrity during operation. This issue is exacerbated in new reactor designs that use liquid metals, such as for example a molten lead-bismuth eutectic, as coolant. In this paper we demonstrate the use of optical fibre sensors to measure the vibration induced by the coolant flow in a lead-bismuth eutectic cooled installation and we apply modal analysis techniques to derive modal parameters of the system. This first-of-a-kind experiment demonstrates the potential of optical fibre based instrumentation in these harsh environments. We focus on measuring the vibration of the individual fuel rods in the fuel assembly, but the technique can also be applied to other components or sections of the installation. We show that the vibration on the fuel rods can be experimentally measured with limited intervention on the fuel pin owing to the small geometry and fundamental properties of the optical fibres. With these sensors, we determine the vibration amplitude and modal parameters of the fuel assembly containing the fuel rods during different steps of the operation of the facility, including the initial start-up of the coolant circulation as well as during continuous operation.