Noise studies during the first Virgo science run and after

The first Virgo long science run (VSR1) lasted 136 days, from 18th May 2007. During the run several noise sources were identified and reduced; this significantly improved the detector sensitivity between the start and the end of the run. We describe three noise studies, showing how data monitoring programs and simple analysis tools permitted the first detection of the noise and provided useful information regarding its origin.