Knocking noise at start up

I have a 99 grand am gt. That makes a knocking noise right at start up, after about 5 seconds of running, the noise quiets. I only notice it at start up.

Recent oil change, and intake gaskets replaced a couple months ago.

What could this noise be?

clicking sound? could be the hydraulic lifter that run out of oil... it is said to be safe to ignore if it is. they click until they fill back of oil.

my dad said its a knock not a click, and he thinks its a bearing?

I had a sharp "clacking" noise, about 4-5 clacks, everytime I started mine up. It was the ABS system running thru its start-up cycle. It did that for several years, then the ABS motor gave out and I replaced it.
Hope this helps.

6 cylinder? Possibly piston slap....