Hi, I got a 2000 Civic DX, 40k. I get a strange metallic ticking/popping sound usually during half throttle when accelerating through 40-60 mph. It doesn't happen when I floor it. It disappears when I reach cruising speed. It only happens when my engine is fully warmed up after having driven 5-10 miles. It doesn't happen when I rev the engine in park. I took it to a couple mechanics...they can't hear it, but they think it's probably misfiring (I thought my car had a knock sensor), and I need a tuneup. I was also told it might be an exhaust leak or my valves should be adjusted. Any thoughts?