, thanks for scene!
there are 2 problems with interior:
1) Post-process exposure 6.7 - it's too hard for rendering sample to operate on almost black lighting and then compensate it in post. Numerical imprecision and other things have more influence at this scale. It's better to operate on exposure at values close to 0 or 1, 2. Don't forget to make Expected Exposure in render settings to be similar to the one in Post.
What I did is to decrease Post Exposure, Increase HDR multiplier and increase the Clamp Direct/Indect values to 20. It's less noisy, but also has different contrast. So need to improve precision in future.
2) Some problems with sampling of your HDR. I don't know yet, but maybe the sampable resolution that we consruct is too low, need to see and not make +1 user setting. Actually I have tested another HDR (from the FB post scene). And it has much less noise. However, it's also bright, contrast (narrow sun), but it's lowres. Lowres/highres of original HDR shouldn't be influential, but the resolution of sampable HDR is.
Dark corners in 2 fixed versions have less noise as I was looking for. So I am calm for that CentiLeo special GI sampling algo. Just need to improve HDR initial procession