loud rattling from engine compartment idling in drive. issues shifting into rev

hard for me to tell. noise only occurs while in drive. no noise when i look under hood in park with engine running. also, my reverse gear problem has vanished once i discovered that my parking brake was engaged...how stupid is that ? i never use it...dont know how it got set. to answer your question, i tend to think the sound is coming more from the underside of the engine maybe slightly to the left.