Welcome to 2CarPros.
Most likely, you have either have a loose heat shield or you have a catalytic converter that has come apart internally. What will happen is this. In park or neutral, the engine RPM's are higher and the engine runs a bit smoother. When placed in gear, the RPM's drop and the engine may run a bit rougher. If it is one of the things I mentioned, they can rattle. When the engine warms up, the metal expands and the rattle goes away.
Here is what I need you to check. First, the exhaust / heat shields. Picture 1 shows the shields.
Picture 2 shows the components which should have a heat shield above them or around them.
Picture 3 shows that catalytic converter, which will also have a heat shield.
Take a rubber mallet and tap on the exhaust at different points to see if you can recreate the sound. If you can, locate where it's coming from
Next, if you can't locate the sound and have no one to put it in gear for you, try turning the AC on and all electrical components such as the fan, lights, wipers and so on. That should load the engine down enough to recreate the sound while the engine is in park. Then you can check it yourself.
Let me know what you find or if you have other questions.
Take care,
Joe
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Tuesday, May 28th, 2019 AT 6:53 PM