From TalkBass Wiki
1) discordant, warbling like tone resulting from excessive pull on the strings at the point of the pickup magnets which produces a string with extended cycle time and uneven oscillaton. Due to improper pickup height or dirty/rusty strings.
2) an eccentricity of resonance which either enhances or damps the note in question. Sounds like a strange mixture of beats because of the periodic diminution and augmentation of the odd harmonics of a string.
