Thanks for sharing the project file. The source of the noise is in the indirect lighting. By adding a Render Layer I was able to save out the direct and indirect lighting elements and easily see the noise pattern.
You'll need to increase the Anti-aliasing value to clean up the noise. This will increase the render time, but you can also reduce Easy Cloud 01's Ray-marching quality from 0.832301 to the default value of 0.5 to help compensate for that.
Since you're using the Path Tracer to render, you might try to increase the Max paths per sample value. This can help reduce render times because the "smoother" sampled result means the anti-aliasing doesn't have to work as hard to clean up noise. (but test it first).
Lastly, if you don't need the Path Tracer for other reasons in your project, you could try rendering via the Standard renderer. The standard render time was 03:52 verses the path tracer 23:50 with an anti-aliasing value of 8 for both renders.
Path tracer render
Standard render
