1999 Buick Century Heavy Hitting engine when starting

Tracking this down could be difficult. Sounds like an internal engine problem at first. However, the long pause coming out of first could indicate a transmission (torque converter) concern. Without hearing the sound I would suggest that you have a reputable technician listen to it and give you his opinion.

If that option isn't available and you're mechanically incline. Try this. Remove the torque converter to flywheel bolts (usually three bolts). Once all of the bolts have been removed you should be able to turn the torque converter by hand. Next, start the engine. If the engine starts and runs without the noise; then the problem is in the trasmission. If the noise persists, then it is in the engine. (there have been cases where the torque converter clutch would remain engaged when a vehicle slowed down causing the engine to cut off or creating a loud banging noise) Post the results and if you need more help, we'll do our best.