  An important goal for LIGO (the Laser Interferometer Gravitational-Wave
Observatory) and Virgo is to find periodic sources of gravitational waves. The
LIGO and Virgo detectors are sensitive to a variety of noise of
non-astrophysical origin, such as instrumental artifacts and environmental
disturbances. These artifacts make it difficult to know when a signal is due to
a gravitational wave or noise. A continuous wave search algorithm, Fscan, and
the calculation of the coherence between the gravitational wave channels and
auxiliary channels has been developed to identify the source of noise lines.
The programs analyze data from the gravitational wave channels as well as
environmental sensors, searching for significant lines that appear in
coincidence (using various thresholds and frequency windows) in the
gravitational wave channel as well the environmental monitors. By this method,
the source of powerful signals at specific frequencies in the gravitational
wave channel caused by noise can be determined. Examples from LIGO's sixth
science run, S6, and Virgo' second scientific run, VSR2, are presented.
