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The 3.4L engine has special pistons that are designed to expand as the engine heats up. When the engine is cold, the pistons make a knocking sound that resembles lifter noise or piston slap. As long as the noise goes away after the engine warms up, don't sweat it; this is normal.
Sounds like excess bearing clearance, when the engine sits overnight the oil between the bearings and the crankshaft is able to run back into the oil pan. When you first start it up it takes a few seconds for the oil pump to fill the spaces back up and you get some clatter.
Hard to say over the internet but I will try to explain. If the noise is a distict KNOCK KNOCK and it sounds like it is coming from the bottom of the engine, then it most likely is a rod or main bearing. If it is a TAP TAP noise that sounds like it's coming from the top of the engine, then most likely a valve lifer is failing/failed.
the fact that it goes away when the car is warm, makes me lean towards a bad lifter. Normaly if a bearing is bad and knocking, the noise doesn't go away.
The KNOCK noise will be somewhat hollow sounding and the TAP TAP sound will be more solid. If the KNOCK KNOCK is solid, then a rod or main bearing has most likely failed to the point that a new/reman engine will be needed.
That is an indication of worn internal engine parts, it quiets down when warm because the metals expand tightening up the fit between parts. If it is a true knock then the parts that are most likely worn and causing it are main/rod bearings, or piston bores/rings.
If the knock goes away after it warms up there are 3 possible causes for this: 1. The pistons are slapping meaning there is piston or engine wear. 2. There is carbon on the top of the pistons. This is the most likely cause. GM says this is not detrimental to the engine. 3. The o-ring between the oil pump screen and the oil pump may be cracked causing areation of the oil. This one was supposed to be there from the time the engine was new but I have seen the problem appear at 50k miles. The oil pan will have to come off to replace the o-ring.
the most common issue is what is called piston slap, GM's official stance is that it will not cause any long term damage, in other words it is just annoying, my Corvette has the same noise, goes away in about 2 miles, had it to the dealer and that is what they said, and i agree with them because it has never gotten any worse after years or hard use/.