Possible Clues:

• What claps or rolls during a storm
• The low part of this low sound comes at the end
• Scary weather report?
• Booming noise after lightning
• Noise named after the god Thor
• Sign of bad weather
• One of these surprises was associated with 17 down
• Loudly denounce the most subordinate
• So much noise from above may get below at last
• Lightning-storm sound