Instead of using the word 'whispered', consider:

• murmured
• mumbled
• muttered
• breathed
• sighed
• hissed
• mouthed
• uttered
• intoned
• susurrated
• purred
• said in an undertone
• gasped
• hinted
• said low
• said into someone’s ear
• said softly
• said under one’s breath
• said in hushed tones
• insinuated