[illinoiswronski]

It depends a lot on the exact sound. "Engine Knock" is normally defined as the sound that an engine makes when you begin to get detonation due to too much timing and/or too low octane, in the old days people called it "ping".

I think it is likely that the startup noise you talk about is called "Piston Slap" which is relatively common in GM engines. I notice it in my car esp during the winter.

If you google "Piston Slap" you will likely come up with a week's worth of good bathroom reading material on the topic. The basic summary that I think you will find is that GM will tell you that it is completely normal and does no harm, and the internet pontificators will tell you that it means your engine will surly melt down to molten slag in the next 100 or so miles. If you read anything significant please post it as I have not read up on it much for quite some time.

FYI, the "shortstar" engine is the 4.0 Liter V8 offered in the Olds Aurora which is a smaller displacement version of the 4.6 Liter Northstar V8 used by Caddy.