I have a 2003 Impala with the 3.4L with about 60k on it. For about the first ~ 15 secs after I start the engine I hear a knocking noise. At least thats what it sounds like to me. About 15 secs or so after that it goes away. If I rev the engine and hold it at say 2800 rpm (I don't have a tech I am just guessing) then it goes away after only a few seconds. I don't remeber when it first started doing this.
Is this normal? Has any one else noticed this? Could it be a problem?

 

I am curious myself if anyone has a response to this! I have a 2004 Impala and it does the same thing. Not even 60,000 miles yet! Should i use thicker old or is this a noise lifter noise?

 

I think the answer is that the engine has hydraulic lifters; after a peroid of time the springs in the lifters gets weak from the pressure of the push rods pushing on them and they bleed down overnight. When you start the car the oil pump pressure fills the lifters back up.
Do not race the engine to make the tapping go away faster that is the worse think you can do, just let it idle until the noise goes away.