The fan clutch is a pretty simple device but can fail.
When you first start the engine cold the clutch is engaged so you can hear the fan blades "grabbing" the air, after a minute or less the noise will get less as fluid(oil) in the clutch warms a bit and clutch disengages.
If you were to stop the engine now and then spin the fan blade it should spin easily.
On the front of the fan clutch is a bi-metal spring, this is a mechanical "temp sensor".
This sensor is warmed up by the radiator, NOT the engine.
As the bi-metal spring is warmed up by the radiators heat the clutch is engaged more so fan blades pull more air thru the radiator adding to its cooling of the coolant.
So the warmer the radiator the more the clutch engages, this is a fluid driven device so it is never mechanically locked or unlocked.
A fan clutch that is not engaging when radiator warms up would cause engine temp to rise when vehicle is stopped or driving at slower speeds, this is because there is less air passing thru the radiator.
A fan clutch that is always engaged would make the extra noise but would have no effect on engine temp except in very cold weather, the extra air circulation would prevent engine from getting up to operating temps.
It is safe to drive, watch for over heating, but it will need to be replaced because in my experience a fan clutch engage/disengage issue will eventually become a bearing issue and fan will start to wobble, this can effect the water pump bearing.
If you feel a vibration at higher RPMs then that has already started to happen.
Last edited by RonD; 05-24-2014 at 11:09 AM.