We present evidence of the excitation of vertically polarised transverse loop oscillations triggered by a catastrophic cooling of a coronal loop with two thirds of the loop mass comprising of cool rain mass. The nature and excitation of oscillations associated with coronal rain is not well understood. We consider observations of coronal rain using data from IRIS, SOT/Hinode and AIA/SDO in a bid to elucidate the excitation mechanism and evolution of wave characteristics. We apply an analytical model of wave-rain interaction, that predicts the inertial excitation amplitude of transverse loop oscillations as a function of the rain mass, to deduce the relative rain mass. It is consistent with the evolution of the oscillation period showing the loop losing a third of its mass due to falling coronal rain in a 10-15 minute time period.