twentysevenlitres

Richard (Audinutt) - are you using the trinary switch as your fan trigger?
I would be wary of slowing down the primary fan - its all very well with the aux fan, as Audi would have done plenty of testing and would slow it for NVH issues.
You'd need to find the CFM rate of the engine fan at idle and ensure your replacement fan meets or exceeds the same rate.
However, if you're doing it to reduce noise levels, I suppose it could be worth the experiment, and it'd be easy enough to find a spare fan resistor from another car. Fair bit of additional wiring though, so have fun!

...might be wise to check whether the resistor needs protection from higher voltage on the low voltage side with something like a diode. Might pull up Bentley and check the wiring diagram...

icrashcars

Although straight moves slightly more air, I doubt it's necessary. Id rather have less noise.