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Abstract Understanding flow dynamics inside centrifugal pumps under off-design operating conditions is critical for pump design and operation. We present a biglobal resolvent analysis of turbulent flow inside a model centrifugal pump at a high flow rate past nominal operating condition. At this off-design condition, the flow near the volute tongue exhibits high unsteadiness, characterized by a dominant reverse flow on its volute side and impingementinduced vortex formation on its discharge side. On the discharge side of the tongue, the impeller-tongue interactions lead to an unsteady boundary layer and large-scale turbulent wake formation. By means of resolvent analysis, we reveal the existence of key modes and frequencies that are responsible for the flow unsteadiness. The optimal resolvent modes have the spatial and temporal scales that agree with the oscillations in the boundary layer, indicating a strong correlation between the impingement-induced vortex formation and the perturbation receptivity. The dominant forcing modes appear on the impeller side of the tongue, and overlap with the region where reversed flow occurs. The dominant response modes appear on the discharge side of the tongue, containing both boundary layer and wake structures. These findings offer insights into the perturbation amplification mechanisms in centrifugal pumps at off-design operating conditions and highlight the sensitive nature of the tongue region, which can support improved pump designs and flow control strategies.