Hello,
I wonder what material is best suited to install a transmitter in. Metal or Aluminum. I think Aluminum is probably the best choice because what I've seen when I've been working for radio, tv and mobile devices for a long time, I've seen that all device boxes have been permanently made of aluminum and not of metal. Has in addition tried to build one in a metal box and got strange noise in the carrier.
So, as I understand, aluminum is a better material for radio waves.
Or what do you say?

Aluminium is a metal.

Steel is better because it is more permeable to magnetic flux and will keep in strays from inductors. Aluminium is only as permeable to magnetic flux as air, so it might as well not be there. In practice, though, aluminium does provide shielding because most of the signal is an electric field.

Aluminium sheet is more expensive than low carbon or mild steel (which is probably what you mean by "metal") of the same thickness but it is lighter, however aluminium also offers some inherent corrosion protection from the oxide layer that forms also instantaneously. This can be grown by anodising quite cheaply - you can do it at home. To protect steels from corrosion you either have to paint them, powder coat them or cover them in something that will corrode preferentially like zinc. Having to further process the steel to prevent (and it is not prevented indefinitely), incurs more cost.

For pirate use it doesn't really matter.

The cause of the noise is probably a grounding issue. Make sure the screws are all tight and the mounting faces are clean and shiny. Sand them with some fine sandpaper if you have to, in order to make them shiny.

I've generally used aluminium cases, because ally is the cheapest metal and easy to bend!

In most cases, I've used Redpoint heatsinks (sometimes my own copy ones, made using sand casting) and Aluminium boxes, folded out of ally sheet. If you buy bending bars, they're trivially easy to fold! In more critical work, I've sealed the ally seams (using a chemical process that I won't reveal here because there's a Patent on it), so that the boxes are actually watertight as well as RF-proof!