Cooling fan mystery

[nick Godridge]

Hi all, can anybody out there explain what seems to be happening at the front of my car.
   Background - early Autocraft car.
   
   I have recently replaced the radiator with a custom made aluminium item copied from the original fitted to my car. Original rad fan retained.
   
   When I climbed in to start the car a few days ago, The bonnet was open, and when the electrics were powered up I noticed what sounded like an electric motor from where the fan is mounted. However no fan rotation.
   
   Car was then started and taken out for some excercise, on my return waiting to get back into garage, fan cut in as normal at around 80 deg.
   
   can anybody out there explain the source of the fan motor noise ( sans  fan rotation ) when engine was cold?
   
   Nick G