I too just bought a Freelander Diesel TD4 SE, 04 plate, that I have had for a week and like you I noticed a loud noise when you turn the ignition on. I have looked under the bonnet and none of the fans are spinning. Tracing the noise it seem to be coming from underneath the rear seats, or somewhere under the car. The sound is more like a pump, I am assuming the diesel pump

(Someone please feel free to correct me

)
I have not had a diesel car before so don't know if this is normal. My old car, VW Passat, used to make a brief noise simlar to this when you turn the ignition on, but the it would only last about a second. I assumed it was the petrol pump pumping a small amount of petrol into the engine for starting. (I'm no expert so not sure if that was the case.)
Is this the noise you are hearing, or in your case is it definatly the fans?
It would be good to compare. As I mentioned before I'm not sure if the noise I am hearing is normal for a diesel Td4.
Any advise would be great.