Abstract Sloshing of liquid in a vessel can cause operational problems, especially when the characteristic frequencies of the sloshing mode coincide with the structural eigenmodes of the vessel suspension system. Inspired by sloshing phenomena in a steelmaking converter, this—predominantly experimental—study focuses on a simplified model system. In place of a complex vessel geometry, we consider a rectangular tank mounted on a spring-controlled seesaw to mimic the suspension system of the steel converter. In the course of experiments, we investigated (a) a collapsing water column in a fixed tank, (b) gas injection-induced sloshing in a fixed tank, (c) initial excitation-induced sloshing in a spring-mounted tank, and (d) gas injection-induced sloshing in a spring-mounted tank. In the spring-mounted tank experiments, we found that the ratio of the mechanical eigenfrequency to the suspension system and characteristic wave frequencies determines the global sloshing behaviour. When these frequencies are similar, beat-like energy transfer occurs between the wave motion within the vessel and the motion of the vessel itself. This resonance phenomenon manifests in a periodically increasing and decreasing load on the suspension system. We substantiate our experimental findings with analytical considerations and unsteady three-dimensional multiphase flow simulations. The numerical predictions correlate well in principle with the experiments with respect to sloshing mode frequencies and the fluid-structure resonance phenomenon, although the sloshing motion is artificially damped.