Q: I have a 2004 Pontiac Grand Am. The faster I go the faster the knocking the slower I go the louder. What can this be? Car does bur

I have a 2004 Pontiac Grand Am that until tonight, with the exception of replacing the battery, has driven fairly well. I started the car and proceeded to put into drive and accelerate to approximately 30mph when I noticed a deep rather LOUD knocking noise. The faster I accelerated the faster the knocking the slower I drove the louder and more intense it seemed to get. It was sudden no build up to this at all. However I did replace the battery just this morning hopefully this has nothing to do with it. Do I need to check spark plugs and possibly get a tune up or could this be more serious like my rods or pistons tapping? What is the approximate cost if it is my rods or pistons and is it a part I can get at a junk yard and replace myself. I have only had this car for a little over a week and it seems to be one thing after another. In a positive light the brakes are brand new!!!! The man I purchased car from did mention it burns oil however but I checked levels are fine. Ideas?

My car has 198836 miles.
My car has an automatic transmission.