Shawn and Bruno,

Thanks for responding! Really appreciate your time on this.

Bruno, you said:

"You have Optimize for outdoor rendering on in thne atmosphere editor, but is this an exterior scene? Looks like inside from the screengrab. Untick it to get a better lighting solution."

Yes, it is an indoor scene, with light coming through a window...

"You have quite a lot of radiosity gain and skydomegain in the scene, and at the same time, the light intensity is toned down, which is a bit weird. Inverting this would reduce the noise.

To get rid of the noise, boost the atmosphere quality (try +1 on a problem area render, to check if +1 is enough without rendering the whole scene), and up the shadow smoothing."

And 

Shawn, you asked about volumetric light, which is what the main light is. I disabled that and here's another render, with Optimize for outdoor turned off, both sky gain and skydome set to zero.

There are no other lights other than the main light. I'm trying for a warm glow such as found in the early morning through an open window.

Not really much difference.

(Sun light (main light) settings: Pure white. softness set to 5, Shadows set 89%. If I set softness to zero, I get some harsh shadows.)

Rendering with atmosphere quality boosted up to 1 now.

...Insert some witty or thought provoking comment here...