Transmission rattle occurs in light-to-medium loaded interactions of loose gear pairs when subjected to vibration. The paper provides solutions for iso-viscous and piezo-viscous hydrodynamic lubricant behaviour that are an underlying cause of gear rattle. The non-linear stiffness and damping characteristics are obtained and their variation under transient conditions under combined rolling and normal approach and separating contacts are discussed. Relationships for lubricant reaction under both lightly loaded iso-viscous and medium loaded piezo-viscous conditions are obtained, the latter for the first time for the case of line contact geometries. The underlying reason for idle rattle, as a significant concern in vehicular transmissions, is explained in a fundamental manner through unstable spectral content of lubricant film surface ripple oscillations. Rattle conditions are reduced under medium loads transmitted during vehicle launch from idle to drive rattle. This transition is known as creep rattle with lubricant behaviour pertaining to piezo-viscous hydrodynamics. In this manner the fundamental role of lubricant behaviour, influencing transmission rattle has been demonstrated, which has not hitherto been reported in the literature.