This work reports on a new Kaplan turbine that experienced an unstable behaviour during its start up. The unit experienced strong sound and vibration in that phase. To identify the origin of this unstable behaviour, a set of sensors were installed within the turbine. Subsequently, tests were performed to determine the response of the different sensors during the acceleration of the machine to the speed no load stage. Following a thorough analysis of the results, the problem was identified as the air admission openings of the head cover and their distribution. To address this issue, a reconfiguration of these openings was implemented and their performance was evaluated. Following this rearrangement, the vibrations in the head cover were significantly reduced and the unit was successfully commissioned.