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Sounds like Rod Knock

Really hoping it isn’t, for multiple reasons, what else could it be?
Knocking noise only happens for a second after start up then fades out as car idles. Comes back after reaching towards 3 RPM.
Noise just started yesterday morning.

So, what sounds like Rod Knock, that isn’t?

Nothing, really except maybe a cracked piston. Either way the engine’s gotta come apart.

Does the knock increase in frequency as rpm increases? Does it sound like a ball peen hammer on the side of the engine block? If yes to both, congratulations, you have a bouncing baby rod knock. Sorry!

Yes and more of a clacking, not quite hammer-ish, more like a kid smacking a couple of 2X4 together.
Figured it was rod knock, what with the oil smoking after I pulled over and popped the cap. Was just hoping this car would be worth more than the trouble it’s given me.
Thank you for confirming, I’ll go look at getting a truck now

A rod knock can be made to repeatedly occur by revving the engine to 2,000 rpm and quickly releasing the throttle. The knock will occur as the rpms drop. If smoke is escaping from the oil fill hole it is very likely to be piston slap from wear or a broken skirt. A look at the spark plugs and a compression check on the cylinders with oily plugs might end all the wondering.

So to check for piston slap, just pull the spark plugs and check for oil? How would I go about doing a compression check?

I had a tensioner give out, Sounded like the worst valve clatter you ever heard for the first 30 seconds at least when cold, been to long to remember what a thrown rod sounds like, proper analysis for the proper cure.

Oil on the plugs indicates compression loss and when you add compression loss to a piston slap noise the probability of piston wear/damage is significant. Any of the chain parts stores have loaner tools and a compression guage is usually among those available. Simple step by step instructions should come with the guage.

A cracked flex plate . . . it will sound like very expensive bottom end problems

it’s confused a few people

If you happen to have a manual transmission, this does not apply

If you have an automatic transmission, you have a flex plate, and it is a possiblity, and it can be checked without removing the engine and/or transmission

A compression testing kit, typically between $25 and $50, will come with detailed instructions.

But what I’d suggest is paying a reputable shop for a diagnosis. There are a number of possible causes for knocking noises (I recently ran into a bad motor mount causing knocking) and an experienced mechanic can tell you what’s causing it with pretty good certainty. Just tell him you only want a diagnosis and not a repair. It’ll probably cost you an hour’s shop time (most shops charge this for diagnosis, and it’s a fair charge), but it’ll be well worth it.

Post the results. We do care.

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Folks can get confused by the sound of severe pinging, thinking it is engine parts knocking rather than what it is, the sound of the gasoline exploding prematurely in the cycle. OP’s description doesn’t really sound like pinging but good idea to eliminate that as a potential cause before embarking on expensive testing or tearing the engine down. Recommend to check the ignition and valve timing, that there are no lean or rich diagnostic codes, and make sure the coolant is running at the correct temperature. Since it came on all of a sudden like, might want to do a drain and fill on the fuel tank too, to eliminate bad gas as a cause.