The number one reason for having noise in a scene, is when a light's "volume" intersects with another geometry. Thea Lights have a parameter called 'Soft shadow - Radius (m)'. It equals to 0.02m and is enabled by default.
When enabled, an invisible sphere is being created with the defined value which is in meters. It is very important to have this number in mind when creating Lights because if the radius is too high, the "sphere" may intersect with any nearby geometry.
As a precaution, the first time a Thea Light is being created, it is being moved away from the point of creation by 1 inch.
In the following image you can see the setup of this simple scene.
In Image 2, the Spot Light has a Soft Radius value of 0.02m and is placed an inch away from a light with a passive emitter. This gives us a clean image.
The noise becomes visible as the Radius of the light increases. (Images 3 & 4)