1. Gentle utterance use to call someone's attention
   - ahem, hem 
2. Exclamation expressing doubt, deliberation, dissent, or hesitation
   - hum, ahem

Encyclopedia: Hmm

• cognisance
• cognisant
• cognise
• cognition
• cognitive
• cognitive content
• cognitive factor
• cognitive neuroscience
• cognitive neuroscientist
• cognitive operation
• cognitive process
• cognitive psychology
• cognitive science
• cognitive scientist
• cognitive semantics
• cognitive state
• cognitively
• cognizable
• cognizance
• cognizant
• cognize
• cognomen
• cognominal
• cognoscente
• cognoscible