Beginner question: vibrato

[TONY ROVELLO <ajr2r@xxxxxxxxxxxxxxxxxxxxxxx>]

Hey y'all,

     I just started about 3 weeks ago on harp.  I'm doing pretty well so far;
I play guitar so the first thing I did was figure out the notes of the harp. 
I know what I can hit and what degree each blow or draw hole is, so I can
navigate around okay, but I can't do much technique wise yet.  

     My big question is about vibrato, and I think I'm talking about throat
vibrato.  Often times when the chord progression goes to the IV chord, the
harp player will hit one note and hold it with vibrato.  Other times, it's
throughout the song.  The best example I can think of is "King Bee" by Muddy
Waters where Cotton holds the 5 draw throughout the I chord of the song, and
he's using vibrato.  Is this throat vibrato, or is it more like a trill where
you wiggle the harp back and forth across your mouth?   What's the right way
to do throat vibrato?  What's the right way to do the trill I'm referring to?

     Thanks for your responses.

     Tony
     ajr2r@xxxxxxxxxxxx