Possible Clues:

• Extraterritorial travellers left behind a sort of sound
• Alan's manufactured part of helmet
• Type of cavity or spray
• Space agency heading for Leo, being nosy?
• Alan's put out by such a sound
• One knows about the sound of this
• Mont Blanc feature?
• German as a language has a certain twang
• Like a twang no Scottish girl produces
• Like Steve Urkel's voice