In this work we have fabricated and characterized GaN based surface acoustic wave (SAW) delay lines grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrate. The acoustic wave velocity of 0th Rayleigh and Sezawa modes, and the piezoelectric electromechanical coupling constant have been measured for different wave numbers in a 2 mum-thick layer. The acoustic velocity resulted to be independent from the layer resistivity, which strongly affects the noise level. Through the introduction of a highly resistive GaN buffer layer, a noise level as low as -70 dB has been measured. This result has been attributed to a reduced coupling between the input and output terminals.