Noise from a source such as an engine or gearbox can travel through a surrounding structure by many routes. It can be very difficult to reduce noise transmission to the environment or to operators or passengers, because many paths contribute to the transmission and it may not be clear which need to be treated. In order to apply effective control measures, it is very helpful to start with knowledge of the relative importance of the various possible paths. We report on a method that allows the strengths of the different paths to be quantified and ranked in order of importance, without having to dismantle or disconnect the system. Having measured these, and then having measured the levels of noise being injected into each path by the engine or gearbox, the contribution of each path to transmitted noise can then be estimated. Finally the noise reduction obtained by treating any desired combination of paths can be predicted. We refer to the method as “Transmission Path Audit” (TPA). The method was applied to an operating wind turbine that was giving noise problems. A TPA was carried out: it indicated that the problem was due to the turbine blades and tower, which between them radiated the bulk of the noise at three different tonal frequencies. Based on these results, damping treatments were applied to the blades and tower, and noise radiation was reduced to acceptable levels. The TPA method has very general applicability, and can be used to characterise noise transmission through a wide range of structures. It is likely to be particularly useful in tracing transmission paths through vehicles, from the power plant to the passenger space. Its novelty lies in its being entirely non-intrusive.