Advanced LIGO and Advanced Virgo have detected gravitational waves from astronomical sources to open a new window on the Universe. To explore this new realm requires an exquisite level of detector sensitivity, meaning that the much stronger signal from instrumental and environmental noise must be rejected. Selected examples of unwanted noise in Advanced LIGO are presented. The initial focus is on how the existence of this noise (characterized by particular frequencies or time intervals) was discovered. Then, a variety of methods are used to track down the source of the noise, e.g., a fault within the instruments or coupling from an external source. The ultimate goal of this effort is to mitigate the noise by either fixing equipment or by augmenting methods to suppress the coupling to the environment.