Related Clues:

• Went (on) monotonously
• Hummed in a dull manner
• Spoke tediously, with 'on'
• Sounded like a didgeridoo
• Buzzed like a bee
• Sounded like bagpipes
• Kept a sustained pitch
• Spoke boringly about getting swamped when wife left
• Risked putting one's students to sleep, say