Hello there,

I own a 2000 Civic DX and when I first start the car up and drive away it makes a loud clattering sound almost like the enging is knocking. if I rev the egine just sitting at a light it doesn't do it only when I accelerate and after 5 minutes or so it stops. but even after I drive for 30 minutes or so and the engine is warm, I can stop at the store and run in for 5 min and come back out and start the car up and it does it again even when the egine is warm and then after I am on the road for a while it stops. I have over 100K on the odometer but this has been happening even since around 70K miles. I give it an oil change regularly ( I even switched to synthetic). I recently changed the fuel filter by the firewall. The only thing I have never done is change the spark plug wires. The plugs yes. the car passes smog no problem. and the car runs great. a little rough idle once in a while when the AC is on but rarely.

 

Have you adjusted the valves? A clattering noise when cold is typically the valve clearance too loose.

It could also be actual knocking. The spark timing is adjustable on that car and should be readjusted at least every time the timing belt is changed.

 

You might also check for loose or broken exhaust heat shields. Another possibility is a bad catalytic converter. Is the CEL ON?