[Len]

The problem may not be in your home it could be in one of your neighbour’s houses.
The front room and hallway in my house have wooden floors and I suspect that all the other houses on my block are the same. So perhaps if something is burning (an electrical wiring fault) two or three doors away then the smoke may well be drifting through the gaps and cracks under the floor boards of every home and gradually finding its way into your hallway through the floor vent next to your front door. Maybe the reason for the odd ball timings of this smell is because what ever is faulty maybe being controlled by a timer or something i.e. central heating system?.
Or maybe one of your neighbour’s has a plantation of cannabis growing in their loft and the number of sun lamps that they are using are drawing that much current through the wiring that the wiring starts to fry. hehe