The effect of heat transfer enhancement in offset strip fin geometries relies upon the repeated interruption of the boundary layers and the oscillatory flow generated by vortex shedding from the individual fins. While the latter phenomenon in most cases does not lead to large-scale flow instabilities, some investigations report a fluctuating behavior of Fanning friction factor f vs. Reynolds number Re in offset strip fin arrays with several relative maxima. The research work presented in this publication comprises an experimental and a numerical investigation of a geometry showing this effect. The experimental investigation is part of a larger study on different fin geometries produced by additive manufacturing. Results are presented for two highly similar offset strip fin test elements with only one of the geometries exhibiting the fluctuating behavior. The geometry of interest in addition shows high-intensity tonal noise emissions in a narrow range of Reynolds numbers. The numerical investigation comprises 2D unsteady CFD analyses of the complete test element with high spatial and temporal resolution, thus allowing to capture velocity and pressure fluctuations. The analyses are able to reproduce the fluctuating behavior of f vs. Re and to predict the noise emission frequencies of the test geometry. Regarding the effect of fluctuating velocities upon Fanning friction factor f, at the relative maxima of f vs. Re the flow field in the offset strip fin array contains areas of large cross-flow velocity gradients which can be assumed to be associated with the increased pressure drops detected at the respective Reynolds numbers.