Loud ticking/knocking/rattling sound when I start/accelerate

Fix your car right away by clicking the link I attached
2001 Pontiac Grand Prix GT Workshop Service Repair Manual

what is your model number is then we can get started then

its a bearing knock

sounds like lower end bearing damage (connecting rod) when oil level drops at interstate speed a connecting rod bearing will seize very quickly thus causing the bearing to spin in the rod and enlarge clearance . that is the knocking sound you here .

If the noise still sounds like it's on top, I'd remove the valve covers and check the rocker arms to make sure they aren't broken or worn. Also crank the engine over and observe how far each travels. If any are different, a lifter or lifter and cam have gone bad.
If your oil pressure is low or the noise is deep inside the engine, likely you have a crank bearing problem. In most cases it's more economical to get a scrapyard replacement rather than do a rebuild.
Before doing anything like that, have an experienced tech listen to it. Only other thing that can make a noise like that is a broken flywheel. A good tech can tell the difference between noises and give you better advice than I can because he can hear it.

How loud is your engine knocking or rattling?

No I would not chase to the pistons, these vehicle were very comon in creating HLA noise hydraulic lash adjusting valves, meaning the valves pump up with oil as the oil gets thinner they dont pump up as fast and create a very loud ticking noise, I would not express a great concern over simplicity, dodge was famous for this, they really were.

Check the Alternator / Powersteering belts .
One of them will be damaged and loose.
Hope this helps.

ROD Bearings DO NOT "TICK" They KNOCK Loud believe Me You Would Know it if it Was A ROD Knocking. I would Advise you Check your OIL Pressure Manualy as the Oil Pump Can Contribute to the Problem you Are Describing

big end or main bearing.