Wandering lines look like a meandering frequency in the time-frequency plots. They can be caused by motors (such as vacuum pumps) that do not have fixed frequencies, or by beats between higher frequency signals that have some frequency variation. Multiple wandering lines can be present at the same time but at different frequencies. Note: This may be related to a very slowly changing whistle / radio frequency beat.
Audio of Wandering Line (with associated spectrogram) Make sure to wear good headphones!