Possible Clues:

• Piece of peach oozing might make you sneeze
• Sound muffled by an elbow, perhaps
• Sound not often made with one's eyes open
• “Gesundheit” preceder
• Sound an allergist hears
• Sound heard before "Gesundheit!"
• Sound of an allergy
• Sound prompting a 'bless you'
• Sound triggered by pollen
• Sound before someone says 'Gesundheit!'