The fans WILL run constantly if the air con is turned on, but if it is turned off, then the fans should only run when needed.
However, the fans are not like old skool electric fans, which were either on or off. Modern electric fans are controlled by the "Coolant Control Module" (CCU) - sometimes a stand alone ECU, or sometimes integrated into the Engine Control Unit. This basically controls the fans on an "intelligent" or "fuzzy-logic" basis. The fans themselves will be controlled to run at maybe 4 or 5 differing speeds, depending on their requirements - from "only just turning", to "sounds like a Concorde engine". Furthermore, the CCU will also "self test" the fans, even if the car is stone cold. This usually happens just after the engine has been started, and basically just runs the fans briefly at low speed to make sure the actual fans are working.
So given the above, do yours sound as though they are working correctly, or are they permanently in "Concorde mode" with the air con off?
The coolant - how low is it going before you top it up? And how high are you topping it up to? You may simply be adding too much, and when the engine gets hot, it pushes the excess out of the overflow. Providing the coolant expansion bottle doesn't completely empty, leave it without topping it up, and just let it "find its own level". My last S4 would be happy at about 3/4 of an inch below the minimum - every time I topped it up, it would vent it back down to its own preferred level. My current RS4 sits perfectly on the max level, and SWMBOs GTI sits just under the max level.
Has your low coolant warning level light/DIS message ever appeared in the dash?
