Sennacherib heard that which king of Cush had set out to fight against him?

Answer: Tirhakah

Warned that Tirhakah king of Cush (KJV: Ethiopia) was marching against him, Sennacherib sent messengers to Hezekiah again.

“Now Sennacherib had been warned about Tirhakah king of Cush: “He has set out to fight against you.” On hearing this, Sennacherib sent messengers to Hezekiah, saying,”— Isaiah 37:9 (BSB)