Wolf tone
A wolf tone, or simply a "wolf", is an undesirable phenomenon that occurs in some bowed-string instruments, most famously in the cello. It happens when the pitch of the played note is close to a particularly strong natural resonant frequency of the body of the musical instrument. A wolf tone is hard for the player to control: instead of a solid tone it tends to produce a thin “surface” sound, sometimes jumping to the octave of the intended note. In extreme cases, a “stuttering” or “warbling” sound is produced, as in the sound example. This sound may be likened to the howling of a wolf. A somewhat similar sound is the beating produced by a wolf interval, which is usually the interval between E♭ and G♯ of the various non-circulating temperaments. (Wikipedia).
