More like a "thock thock" sound. Any thoughts? Goes away once engine reaches operating temp.
Common small block problem. If I remember correctly GM changed piston design going with a smaller piston skirt. So tendency to develop a piston slap when the engine is cold.
Being the noise goes away after the engine warms up is enough to confirm that piston slap is what you have. Save your money buying any listening tool. And bore scope's are not going to show you anything concerning piston slap.
I switched from 5w 30 oil to 10w 30. That helped reduce the noise and shortened the time needed for the noise to go away.
Then I told the wife that she must warm up the engine and wait for the noise to go away before she puts the suburban in gear.
At least 25,000 miles with cold engine piston slap. I am living with it. And I know of others that have done the same.
And I would not call that sound a knock. Main bearings, connecting rod bearings, that's a knock.
