Nissan Repair: is my car over heating??, nissan sentra 2001, nissan sentra

Question
Hi Von,
Since i'm not technically savvy with cars, I would need your help. I have a Nissan Sentra 2001 GXE 1.8. I noticed for the past couple of days that my radiator fan is making a loud sound. When i open the hood, i notice that the radiator fan is spinning really fast and generating a lot of air, guess its blowing out hot air. I am not sure if this might be caused by me adding coolant the week before. When i see the coolant level it is a little above the MAX level. Is this happening because of this?
Secondly, when driving on the highway, the temperature gauge on my dashboard seem to be normal(slighly lower than the mid point between Cold and Hot), and i noticed that when i slow down in traffic or come to a full stop, the temp goes a little above the mid point of Cold and hot, towards the hot region.
What could be the problem. And i don't know if this might help you, but when i turn my ignion to the 'on' position, i hear some kind of water sound going somewhere. Sounds like someone is drinking from a straw. But this sound very faint.

I am slightly worried, is there anything you think is causing the problem. My friend is saying it could be a faulty thermostat.
Your help is highly appreciated.  

Answer
Rex,

First I would change the thermostat and the radiator cap.  Also, change the antifreeze out with a 50 / 50 mix of distilled water and antifreeze.  Do not back flush the radiator as it will blow the heater core.  Just drain and fill with water a couple of times to get most of the sludge out along with the old coolant.  Then add the 50 / 50 mix.  Your car should be fine.  Remember that the car needs to the coolant changed out every two years.  This might be a good time to check and change the belts and hoses.  After this you should be fine.