Although wind turbine noise is mainly dominated by aero-acoustic noise, mechanical noise, coming from gearbox or generator, could—especially when it contains audible tonal components—result in nonconformity to local noise regulations. To reduce the mechanical noise from the gearbox, focus is put on first time right design. To achieve this, simulation models are being used earlier in the design process to predict possible issues. This paper starts with a short overview of the used model and gives additional insight in how forces from planetary gear stages should be introduced in the flexible housing. Main focus of this paper however is the approach that is being used to validate such a complex multibody model of a wind turbine gearbox. The validation approach consists of five levels: (1) individual components, (2) assembly of the empty gearbox housing, (3) the assembled gearbox, (4) the gearbox on the end-of-line (EOL) test rig, and (5) the gearbox in the wind turbine. This paper focuses on the experimental measurement results, the correlation approach for such complex models, and the results of this correlation for the first four levels showing the usability of these models to accurately predict the modal behavior.