Rat-tat-tat when first starting the engine cold

[Guest bastaad525]

Okay I don't know if my engine's been doing this before or for how long, but I just noticed it a few days ago. When I start the engine cold (and I mean it has to be completely cold, pretty much only when I start the car in the morning after it's sat overnight) there is a semi loud sound... kinda hard to describe... a friend of mine came the closest I think when he said sounds like hydraulic lifters that are old and take a few seconds to get the oil in them... so I guess kinda a loud valvetrain sound though I'm not sure it's coming from the top half of the engine. Anyways this mystery noise only goes on for about 1-2 seconds after starting the engine, afterwards it sounds perfectly fine. By the way I know I mentioned the hydraulic lifters but this is on an L28 engine with an N42 head... I'm just using that description for comparison. The block has only about 20,000 miles on it and the head is fresh with only about 1,000 miles on it.

 

Any ideas as to what this sound could be?

[Guest bastaad525]

Just wanted to add, I went out and started the car this morning and this time it didn't make the sound at all. I was thinking it may be something like rod knock or a piston pin hole wearing or something, but if that was the case wouldn't it keep going until the car was at least a little warmed up? Anyways I've been kinda testing it and I figured out that it doesn't always do it, even when the motor is cold, but only if it's cold AND I'm giving it any accelerator when I'm starting it.

[Guest Anonymous]

How many miles on the oil pump? you might want to check oil psi. with a mechanical gague.

[TomoHawk]

Certain cheap oil filters will drain overnight, which means that the entire engine is dry in the morning. This happens because the cheaper oil fil;ters don't have a "drain-back flapper" which lets all the oil drain out of the filter. So in the morning, the oil filter is empty, and the rest of the engine is too, so the oil pump has to pump about an extra 1/2 quart of oil before it can pump oil into the crankshaft and pushrods, which means they ar running dry( bad) until the engine gets oiled up. The 'hammering' you hear is probably the dry pushrods or the dry rod ends banging because there is no oil in there for lubrication.