Where to mount Air Lift compressor on 07 Ram 2500?

I put mine inside under the rear seat.
Noise is not an issue whatsoever, you only run it when you're airing up your bags, it's not like it's on all the time. It doesn't run long enough filling your airbags for air circulation to be an issue.
Completely out of the weather and water is best, that's why I put it inside.
Plumbing isn't a nightmare. It's easy.
Having said that, I also have one mounted underneath on the frame that's been there for years and it still works.