Having a similar problem so did a bit or "Googling". Seems the problem could be to do with surfaces that scatter light in a non-predictable way. The wikipedia entry on path tracing mentions three areas which cause problems.

• Bright, sharp caustics (also mentions specular and glossy in other articles)
• Subsurface scattering
• Chromatic abberation

There is mention that using the Metropolis algorithim might help.

Descriptions were way over my head but as a thought if the scene was entirely enclosed by an object that emitted some light would that reduce the noise problem (i.e. all rays resolved to a light source) - just speculating.