Knock at startup? HELP!

Ok so I have a 2000 alero with the 2.4L. and when I turn the key I get 3-4 loud knocks just as the motor starts to crank. it starts fine. runs smooth its def. from the motor not a backfire. its rather loud and scarey. ...Just as I start to turn the key KNOCK KNOCK KNOCK start...I can't track it down and it just started out of the blue.most things are new, CAT, plugs, boots, oil, all filters, air,fuel,oil. and a bunch more things I replaced over the last year or so. What could cause this to happen so suddenly. Everyone I ask talks about knocking after its started and running but it has NO KNOCK or rattle or anything while its running it's just as its cranking over. please help?

Also should stress the word knock not tick or ping or clank. it's def. a knock almost a thump.

I had the same experience when I bought a used 2000 Alero 3.4L; when I started it up there was a series of hums, knocks, ticks, etc. After awhile I just got used to it. My guess is that your knock is the ABS system doing some self-test. I think the ABS
motor/controller is under the battery, maybe you could listen in that area.

I think it would be interesting to list the noises Aleros make on Key-On, then start-up. There's the fuel-pump hum, the ABS knocking, the HVAC vents adjusting, the A/C compressor coming on (if A/C is set to On, which mine always is) any others?