Possible Clues:

• Expression of disbelief
• Bullish comment?
• Audible expression of contempt
• Advice from 23-Across?
• Relative of a harrumph
• Emulate Johnny Depp in 'Blow'
• Show of incredulity
• Force air out of the nose
• Sound akin to 'Harrumph!'
• Sound indignant