1992 Oldsmobile Silhouette can't find noise

Engine Mechanical problem
1992 Oldsmobile Silhouette 6 cyl Front Wheel Drive Automatic

I have a rattling that seems to come from the front lower part of the motor. It rattles and sounds like a loud knocking until you step on the accelerator then it stops completely. it immediatly starts again when you let up on the gas. Please could you let me know if you have any ideas.