Possible Clues:

• Experience fear as Mike interrupts singer
• Head of Music enthralled by singer's vibrato?
• Shake this three times about a thousand
• Shake with fear or excitement
• ...it should wobble three times as much, with added mass
• Minute into really stiff drink, get the shakes
• The way more than twice about a thousand is enough to shake you
• Choirboy about to open Mass appears to shake
• Look nervous as chorister in Mass? The opposite!
• Shake - quiver