BMW: BMW overheating, ecu computer, coolant level

Question
Hello, I have an 2006 BMW 525xi and I got a low coolant level message.I took it to the dealership and they said I needed to replace the radiator and the sensor. They were very rude and since I was a women acted like I could not read or understand anything. I checked the coolant level and it was fine. A few weeks later I got the overheating message. I had the car towed home and again the coolant level was fine.
Any ideas if this could in fact be the radiator or something else.
Also, when the car is idling the fan makes a strange noise. It sounds like it get very loud and then quiet, then loud then quiet. I don't think this is normal but honestly never stood outside of my car while it was idling.
any input would be greatly appreciated.
Thank You

Answer
Hi Michelle , first of all any rude behaviour by the service dealer can be reported to BMW helpline as this kind of service is unacceptable . Evenif you can't make out the bonnet from the trunk thaey can't speak to you like this . Secondly what they suggested is absolutely false , you just need to get the sensor replaced as there is no physical loss of coolant . The engine fangets instructions from the sensor which is faulty , that is why it switches on when the sensor tells it to and swiitches off onces the car's ECU (computer) checks it for itself .