[A26B]

I slowed the video down to 50% speed & counted 40 noise events in 6 seconds, which translates to 400/min. A lifter or exhaust leak timing would equate to 800rpm, a normal for a cold start rpm after a few seconds.

Just my thoughts....
Considering your very cold ambient temperatures at start and the metallic sound I am leaning towards a collapsed lifter. Cold temps increase oil viscosity which in turn impedes oil flow. The orfices providing oil to each lifter are very small. At start up, with very cold oil, the engine could run for a short time with minimal oil pressure to the lifter itself. During this time, the lifter could "bleed down" from the pressure exerted during a valve opening sequence..

Watch the Cold Weather video at the bottom of the page & imagine non synthetic oil trying to flow through a pin hole size orfice to the lifter.

https://mobiloil.com/en/article/why-...n-cold-weather


LT5 engines have some unusual characteristics. Putting an engine back together without collapsing the lifters first, can result in valves being held open and no compression until it "bleeds down". So the simplest cure is to be sure all lifter are collapsed first. They will then "pump up" to the proper height for that particular valve stem & cam lobe.

Since the lifter noise started, I am assuming the engine has never actually warmed up.

If possible, you may consider going to a lighter viscosity (0-20), fully synthetic oil. Before draining the 10W30, non-synthetic oil currently in the engine, warm your garage up (or wait for spring) for an extended period to get the engine warm and let it drain for 24hrs before refilling with 0-20 synthetic.

After replacement with the low viscosity synthetic oil, the lifter may "pump up" again and it may not. Keep rpm low & allow engine temps to reach normal, but pay close attention to the noise. If it increases, shut it down.