Pajero 3.8 MIVEC fan

[SimphiweB]

Some advice please: I have a Pajero 3.8 MIVEC, 2008 model, petrol, swb. The engine fan is always on, even on engine start up, and I assume it stays on all the time. When driving at speeds of about 100km/h and above, there is a windy vibration from the engine which has a deep cyclic motion. I have changed the pulleys, belt and checked all that I could. I have had the differential bearings done too. I suspect that the fan may be the source of noise but I'm not sure. Is the fan supposed to be running all the time in this car even when teh engine is cold ? It's not an electric fan, its mechanical i.e. its connected to the pully which is driven by a fan belt. No perfomance, issues the car drives well and it does not overheat.

I had assumed that the fan should only come on at certain temperatures, or this is only applicable to electric fans? Can a stuck-open thermostat cause this or a faulty temperature sensor? Any advice please on what to check.