It is either noise from indirect illumination, or direct one (enviro light), or refractive or reflective caustics.
First one, indirect illumination kind, should get better with increasing amount of PT samples (Usually to something between 32 and 64).
Second kind... direct illumination, will get better if light samples multiplier is increased (usually to something like 8).
Third, refractive caustics, can be minimized by either using hybrid glass mode and/or lowering MSI value.
Fourth kind, reflective caustics, can be again minimized by lowering MSI.
I would bet in this case that it is first or second. I recommend this:
Make a small region (150*100px at most) in the problematic spot
Set time limit to something like 20 seconds
Render and save the picture.
Increase PT samples, render again, save the picture, and compare. If this makes some noticeable difference, then it could be indirect illumination noise.
Revert PT samples back, increase light samples multiplier, render again, and compare with original. If this makes some noticeable difference, then it could be direct illumination noise.
Revert light samples multiplier back, select all glass object in scenes and hide them. Render again, and compare with original. This time to see if the problem is not refractive caustics. If this step has a huge impact, then it is probably glass.
If the noise is still there, and hiding glass did not make any significant difference, then unhide glass, lower MSI to something like 10, and render again. If that makes significant difference, then it is unclamped reflection caustics.
If none of these makes significant difference, then its just probably not that easy scene to render.
Other than that, it could be for example noise from bump map with way too low fitering. Or there can be a very little hole in the back room somewhere, where little bit of direct lighting leaks through from the environment and causes some noise... or there is always small possibility of it being a bug ;)