I have a 1999 Accord Coupe 4-cyl VTEC manual transmission with 90,000 miles. Recently, I noticed a pronounced deep "knock" that is only audible at the instant the engine cranks. This noise is not audible when the starter is spinning the engine and neither is it audible when the engine is running, but can be heard during that fraction of a second when the engine ignition takes over for the starter and the engine actually cranks. This isn't valve clatter or rod knock ... I'm familiar with those. A friend said that he thinks it's the catalytic converter gone bad, but somehow that doesn't make sense to me. Any thoughts from forum members?

 

If the starter is still spinning there is no exhaust. No exhaust and the cat isn't doing anything therefore it can't be the source of your problem. I've never heard of any "knock" from a cat especially since it just sits there and oxidizes the exhaust as it passes through. You might have a problem if the Cat heatshield has broken off from the bolts, which happend in my mom's 97 LX. A little bit of mechanic's wire fixed it right up. That is more of a rattle sound. And if your cat was bad, you would have a check engine light.