If your building is all white (no glass), secondary rays do absolutely nothing for you but slow your rendering down, as they only are visible in reflections and refractions… but still calculated if you have rays set for them. 10 for secondary is often fine if you have glass.
If you want the occlusion look (cloudy day lighting) you will have to have some form of Sky Light, otherwise all you are getting is a bounced light from a standard light which is using Reverse Illumination. Sometimes having a standard light using GI helps with noise from GI Sky Lights. If you are only using Gi Sky Light, then you can turn off Reverse illumination bounces, as they are not used.
When you added dicer the problem probably got worse, as GI calculates between sample pixels which relate directly to your geometry.
With stills, people report getting better results with 32X32 and increasing ray count, as it gives a smoother result (by not being as detailed, it’s often brighter as well. Not as accurate though.) With animations people report get better results with a denser 1X1 sampling level and increasing ray count, as it is more accurate, so the results line up better between frames.
If you double the pixel size (ie 800X400 to 1600X800) of your rendering, does it go away?
You should try adjusting the color tolerance (5-10), and the details factor (1). These also can help. As well decreasing in the cone angle can help, though it looks like it has more contrast.
The best way to see where the noise is coming from is see what is being sampled with ‘show samples’ turned on.
You might get the GI tutorial at Paralumino, from Dave. He has a lot of good tips.
Let us know what worked for you when you get it.
Yon