What cause a knocking noise 5 to 10 minutes after first start in the morning? on 2006 Buick Lucerne

There are three metal knocks, in quick succession, resembles radial tires going over a crack in the road. Happens as fast as you can read knock, knock, knock. It is solid sound, not a tinty and nothing else happens with the engine. No hesitation or drop in RPM.

Asked by for the 2006 Buick Lucerne
car needs to be looked at to determine noise
First of all, I have checked several responses from you on this website and you rarely give advice except for something similar to what you posted here. Second, I checked with the Cadillac forum (same engine) and found out a service bulletin for this problem. It states the knock is caused by the engine heating up before the cadalitic (spelling ?) converter. The system is programmed to puff 3 to 5 hot blasts into the converter. It says it will sound similar to what I stated in my question.
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This is common for most engines when first started after sitting for a period of time. When a engine sits not running the oil drains back down to the bottom. When it is started the engine takes a second to pump the oil from the bottom back through the lifters in the top off the engine block creating a ticking or knocking sound. Adding a quart of Marvel Mystery Oil helps with this issue, find it at auto parts stores anywhere.
I checked with the Cadillac forum (same engine) and found out a service bulletin for this problem. It states the knock is caused by the engine heating up before the cadalitic (spelling ?) converter. The system is programmed to puff 3 to 5 hot blasts into the converter. It says it will sound similar to what I stated in my question.