Humming sound but stops once it is in neutral?

So, this can be the engine or transmission mounts, it could be part of the exhaust system is contacting the frame. I would put the car up on jack stands or a hoist and then have someone in the car as they produce the noise follow it to its origin. It could be the transmission itself. Lift the hood and use a flashlight to help find what is making the noise. We need more information to be able to help you. Please go over this guide:

https://www.2carpros.com/articles/jack-up-and-lift-your-car-safely

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