what causes the rattle or knock in engine when car is cold. it goes away when car warms up.
Most likely the common fault with some generations of GM engine…piston slap.
When first fired up, the pistons bounce off the sides of the cylinder walls (Little too much tolerance there) until the engine oil warms a bit to properly lube the cylinder walls.
Nothing to be worried about as no damaged is caused by this, but the engine can be tore down and fixed.
Our resident techs here can explain what has to be done to rectify the slap.
The 5.3L V8 in my '02 Tahoe and the 3.4L V6 in my '00 Silhouette both do a tap dance when warming up, but it only last for a couple of minutes.
My newly acquired '04 Toyota Matrix 1.8 L four banger doesn’t. Nice.