A rattling noise coming from the engine.

A rattle could be a lot of things. Many auto parts stores will do a free diagnostic readout of the code causing the check engine light to come on.

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I suggest you have your car checked as soon as possible - while a rattling exhaust isn't desperately urgent it could be the cat breaking up which might later restrict the flow of gases. Most importantly excessively loose timing belts or chains tend to make most noise when the engine is idling. If the noise is being caused by this delay could be very expensive.

it could be a wrist pin, and im not sure if those amc's had hydraulic lifters but sometimes with old lifters when the oil pressure is lower at an idle the lifters arnt filling up all the way so they develope a little valve lash, and when the rpms are raised the oil pressure raises and fills the lifters. you could try chasing the noise down with an automotive stethoscope.

IAC (IDLE AIR CONTROL) should be replaced. You will be amazed at the difference when you change it.

lots of parts, alternator, water pump, a/c compressor lower area . many of them are on the serpentine belt. the pulleys and tensioner can make rattles. you need to confirm which part has the rattle or vibration, use a stick or screw driver. touch one end on the part and the other near your temple...any noise/vibration coming through?

fiind which one it is. lower noise on the crank can also sound like its coming from up front and that's a deep rattle.

this is easy one if you put the a/c on do you hear the rattle if yes then shut off the a/c if the noise go away then it is the compressor

Try checking the heat shield on your exhaust system. You should have one over the manifold and one over your cat converter under the car.

Glazed alternator clutch plates? Kind of a ratcheting sound? Common when oil wasn't changed or sitting unused too long.
1. Easy fix - drain oil, remove filter. Add diesel fuel to crankcase. Turn engine over. let sit and drain. Refill with new nonsynthetic oil and replace filter. Warm up. Drain, remove filter. Refill with fresh nonsynthetic oil, and install new filter.
2. Harder, but better. Remove plates and clean or replace them.

If this sounds like a tin can rattle it is likely a loose heat shield on top of the catalytic converter. They come loose after time and corrosion take their toll.