Plastic lenses believe it or not are permeable to moisture. Halogen bulbs create enough heat and IR energy to drive the moisture out. Many OEM lamp assemblies have channels or coatings on the inside to channel the moisture to a place where it will not be seen. HID should generate enough heat...
I have something very similar. Sporadic. Thought it was the brake pedal rattling at first. They have started replacing trim parts in the driver footwell....