It's my Accord with 200k miles on it. When I start it in the morning, it idles at about 1500 rpms without any knocking. Then as the rpms go down, it starts to make this cyclical knocking sound at exactly 1250 rpms. At about 1000 rpms the knocking sound goes away and I don't hear it again until the next morning. I use 10w-40 oil. What could the problem be?

 

Wrong oil for starters

 

No, the oil is okay - it's an old engine, so it needs heavy oil.

 

your engine may me knocking from having thicker oil than it should, or it might be something that has led to having thicker oil in your engine. the knocks go away or lessen when the car is warmed up, because oil gets thinner at higher temp. this is why you use thinner oil when its colder and thicker when its warmer. i bet if you drain and use the correct oil, the knock may go away completely. damage can occur though from using the wrong weight of oil. its common sense...