Unsteady shock–boundary-layer interaction on the lower surface of a transport-aircraft wing can be caused or amplified by ultrahigh-bypass-ratio underwing nacelle installation. This work analyzes the resulting buffet dynamics on the Airbus XRF1 configuration at a Mach number of 0.84, a Reynolds number of 3.3 × 106, and −4∘ angle of attack using scale-resolving delayed detached eddy simulations and unsteady pressure-sensitive paint measurements. Coherent structures are extracted employing a data-efficient multi-taper spectral POD. Dominant modes occur in the Strouhal number range St ∈ [0.1, 0.3]. Surface modes reveal wave-like shock motions that originate near the pylon–wing intersection and propagate inboard toward the fuselage. These shock oscillations are linked to unsteady flow separation downstream of the shock. Additional dominant modes show spanwise oscillations of the separated flow region and shock oscillations phase-linked to shear layer instabilities. The modal analysis of volume data reveals pressure waves connected to these modes traveling upstream above and below the wing.