1992 Subaru Legacy Horrible Rattling Sound

Logic would dictate it has something to do with load on the engine, and that would include loose torque converter bolts, but those will usually make noise all the time. Loose vibration dampers can make a knocking noise but that shouldn't be affected by shifting into gear. Luckily you can keep it making the noise so you can search for the cause.

You will likely be able to find it with a stethoscope, otherwise there is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.