It is that the oil hasn't had time to get circulated to the top end yet. shouldn't be a problem if it goes away after a minute or two. Mine used to do that when it was <35 outside for about 25-30 seconds when it was first started. Then I changed from 5w-30 to 10w-30 and it stopped. they say heavy weight oil usually does that due to viscosity but heavier weight stopped that for me.
I use pennzoil conventional

 

even though the dealership I work at swears by that stuff I would never put it in my truck.

 

i went down from 10W-30 to 5W-30 and it has reduced the frequency that it knocks, like every other cold winter start instead of all the time.

Its definitely in the bottom end, but I cant tell high or low. It usually doesn't last long enough for me to pinpoint it to a specific cylinder either.

 

That slight knock appears to have something to do with the oil pump shaft. Mine has done it every 40 degree or less morning since the day I bought it. I bought mine in December of 2002 with 110,000 miles. I ONLY use straight 40 weight oil 12 month of the year. The previous owner probably didn't and it did it with his oil in it the first morning. Before that I had an 88 and that Blazer did it too so I was aware of the sound. The slight knock has never gotten any louder or worse and it only lasts about 1 minute, less if the temperature is higher, never does it in the summer. I still get real strange looks from my mechanic every time I get my oil changed because I have to bring my own oil as he does not have straight 40 weight oil in stock. I still pay him the same amount for the oil change and it doesn't bother me because I want the heavier viscosity oil. This is the 4th vehicle that I have only used straight 40 weight and the engines last a long time. I just turned over 332,000 this week. If that knock was anything serious, I'm sure I would have blown my engine by now. Four New England winters and still going strong. Eleven winters for the truck over all.

I am actually glad to hear that other people have the same noise that are using much thinner weight oil. We had 3 mornings this week in NH at 25 degrees and the Blazer turned over like it was July 4th. It does get a little hairy around January when we get those below zero mornings, but so far so good. I've replaced the starter once in 220 miles, about average I'd say.

 

ok, so at least I'm not the only one. Maybe its a design flaw in the older oil pump shafts that theyve fixed for the newer models? Blasir, yours has the exact same symptoms, to the same degree, 40 or less; and never does it in the summer.

 

You might try some Lucas Oil Stabilizer. The synthetic formula is about the same consistancy as 5w30. Lucas makes some GREAT products. Their fuel injector cleaner is awesome.

 

Did you ever sort out your knocking? Mine does the same thing and I got over 140,000 miles on it. Goes away in about 10-20 seconds though.

 

Piston slap? My LS1 sounds like a diesel when it starts (using sarcasm) but its noisy till it warms up. Everyone on LS1tech says its normal. Guess its just notorious in those cars

 

my motor is from a 2001 so I don't think it was a design flaw that has been fixed...my dad's truck has a 350 in it and does that on cold mornings also.

 

my 93 4.3 w had a bit of sludge in one pushrod once cleaned it was fine, also if u have a block heater use it, it also saves a bit of gas