My car developed an annoying rattle. I took it to the service station. The mechanic looked under the hood. He couldn’t find what was wrong. (compound-complex sentence)

My car developed an annoying rattle. I took it to the service station. The mechanic looked under the hood. He couldn’t find what was wrong. (compound-complex sentence)

My car developed an annoying rattle, so I took it to the service station. The mechanic looked under the hood, but he couldn't find what was wrong.