Underwater noise monitoring of a tidal stream is an important component of any Environmental Impact Assessment to assess the potential noise disturbances that a tidal turbine may have on marine life. Before monitoring operational noise, it is crucial to characterise ambient noise at different period of a proposed site. In May 2011, an acoustic survey of Ramsey Sound was undertaken over two entire tidal cycles. Ramsey Sound is a proposed site for the installation of a tidal turbine. It is an open channel measuring 2.5km long and 1km wide, varying between 25 and 70m deep and subject to tidal streams up to 4m.s-1. Continuous recordings have been carried out from 1Hz to 22kHz. The methodology used to monitor ambient noise was to immerse the hydrophone over the board and let the boat drift with the current flow, which allowed the reduction of unwanted noise such as water flow noise, cable strum. This paper presents the impact of the tide, the biological and the physical noise on the ambient noise. Sound Pressure Level during neap tide evolves between 95 and 125dBre_1μParms. During spring tide, background noise SPL is higher and evolves between 100 and 127dB re_1μParms.