The objectives of this work were to establish a method to diagnose the source of gear rattle and to evaluate the rattle objectively. The methods are described in detail, applied to two passenger cars as an example.Investigations were conducted into transmission rattle under transient conditions. By analysing the transmission casing vibration with respect to the engine flywheel angle, and presenting the data in the form of contour maps, it was shown that the two vehicles had different characteristics of gear impacts. Further measurements of the angular motion of each gear revealed the impact conditions at the input mesh in the transmission largely controlled the character of the rattle and were fundamentally different between the two vehicles.A rattle index was developed, based on the casing vibration under transient driving conditions. Multi-dimensional scaling was used to ‘extract’ the principal dimensions from the subjective responses and two physical parameters were used to formulate the index.From this research it was concluded that detailed analysis of the gear motion and casing vibration is an effective tool to diagnose gear rattle and identify the cause of the rattle. A rattle index generated from two physical parameters correlated well with the results from subjective tests.