Based on the level of respect or formality, you can say "That sounds like thunder" in Korean in 3 ways:

• Informal: cheondung sori gata (천둥 소리 같아)
• Formal: cheondung sori gatneyo (천둥 소리 같네요)
• Very formal: cheondung sori gatsseumnida (천둥 소리 같습니다)