Depending on your location, the tempeatures are dropping. This alone will cause internal engine parts to shrink. Its not a dramatic shrink, only in the 1000ths of an inch range. So clearances between parts get a little bit bigger.
The rattle should disappear as the oil begins to refill the circuit and as parts warm up.
After start up, just sit there for a moment or 2 idling until the "rattle" begins to subside. Then drive off, go easy for some distance. No WOT shenanigans. No need to sit there idling waiting for the engine to reach operating temperature.
One more point......unload the parts cannon. I doubt if there are any problems.
BTW, this is normal for all engines. The "rattle" will likely increase the longer it sits and the colder the temperature.
I had an ol' Buick in college. When temps dipped under 0 degrees, at start up it sounded like the 455 was gonna jump out of the car. But I'd give it a minute for a calmer sound, then drive off.