I suspect it to be same as in case of "noise visible in volume coming from mesh-lights close to source" (it's reported).
So with that in mind I modified most of it. When you'll analyze the scene you'll also notice other changes: reduced polys of mesh lights to single quad (note that every triangle is a source of light); sphere isn't Standard, Perfect, nor subdivided anymore, but just icosahedron @100 (less polys, no poles, less pre-comp., still looks perfect); "Wax Fixed" has Layer removed (it's risky to use it w Corona); Frame Buffer Type is set "None" (faster, better for post-pro & OIDN; Highlight Clamping set to "1" (removes excessive bright spots/fireflies, also lowers noise;), Adaptive Light Solver "Off" (precaution measure), Adaptivity (Disabled), Enviro Sampler "Faster"...
I just tried again w/ Adaptive stuff back on and it renders fine. Highlight clamping set back to "0"also shows only minor change... So play around with settings (in bold) and see what change will bring noise back. Learn some and keep us posted.
And in future endeavors just remember of KISS principle (Keep It Simple Stupid).
:)
Enjoy