Well I m no corona expert by any means but to me this looks like GI noise. Therefore, you could try increasing GIvsAA samples to 32 (from 16) for example. Then you could lower LSM from 2 to 1 because at 32*1=32 you get same result as with 16*2=32. Increasing GIvsAA will increase render times but it should get rid of noise. Maybe try 64*0,5 combination if 32*1 doesn't work. Don't go over 64.
You could also try to slightly reduce MSI (Max Sample Intensity) from 20 to something like 18 and that should also help reducing noise. Don't decrease it too much (to values below 10 or so) or you will lose brightness and render will not look realistic
