Vibrato is a change in pitch, tremolo is a repeated note, or rapid change in amplitude of a note. Often, I hear, particularly with vocals, a rapid change in tone, which is neither of the above. The pitch stays the same, and it's not a re-iterated note (or a trill), but a different sort of embellishment. Almost the same as a classical guitar vibrato, which changes the tone, but that also very subtly changes pitch. An approximation on guitar may be rapid use of a wah pedal - the tone of a note played is fluctuating, only. Is there a term for this, as it would appear to be a common vocal approach to, in particular, longer notes?
I sort of disagree with the first statement: what vibrato actually is, depends on the instrument group at hand. For (at least: wood-) winds it is a variation of support, which would mainly translate to a variation in volume (but admittedly also some pitch variation as well). On strings as violin situation is opposite, that main effect is a variation in pitch.For human voice the observed sorts of vibrato are so various, I would not attempt a definition there.
As mkingsbu comments, brass players can evoke pitch variations by their lips; a brass colleague stated, that vibrato is mainly amplitude variation with some pitch enclosed, so maybe different techniques lead to varying results.
A technical term would be amplitude variation; modulation would also be appropriate for speech, but has a different meaning in music. Articulation would surely cover it, but is a bit vague.
Well, your comparison to a guitar vibrato does sound like a proper singer's vibrato (which is mostly a self-sustaining effect of a relaxed throat).
There is also the baroque trillo but it's far less subtle than your initial description and a rapidly interrupted phonation, so not really related to vibrato all that much but more to a guitar tremolo.
Tim, I know exactly what you're talking about. It's called a timbral trill. This is a term employed by theorists and composers to describe sounds in which composers use alternate fingers on woodwind and brass instruments to trill between fingerings of the same pitch on a given instrument. Though it is the same note, there are minute variations in how it resonates through the instrument and therefore minute changes in microtonal vibrations, thus affecting subtle variations in the sound. It is much less commonly associated with the voice, but, if a particular singer had the facility to change the quality of their voice, a "vocal timbral trill" is certainly possible.