Could be the exhaust system a pipe, mounting hanger, heat shield, to a rotational idler puller under the hood try and isolate it.Does it increase with engine speed? or with your foot on brake shift into gear any change?
Engine making rattling noise while idling may not be good. Need more information.
with the car off and and the exhaust cold go under the car and tap on the exhaust with a rubber hammer or a dead blow hammer if available and see if one of the heat shields are loose. sometimes you can just bend them a little so they don't rattle
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