My car makes a rattling sound under the car

Hey Thealmeidas!
Yep, there's a good chance some part of the exhaust system has come loose and is rattling at certain RPM's. Start fault-tracing by checking if any part of the exhaust seems loose to come in contact with anything, or if there's any loose clamps or similar parts hanging onto it. Another quite common reason for rattle is that some part of the exhaust manifold, such as a bolt or hose/pipe has loosened (often due to rust and/or corrosion).
Another possibility is that it's somehow related to the transmission (since you mention putting it into gear). If so that would be another subject.
Good luck!