I know that others with A5's have had fan failures as early as 36K miles. My A5 is at 102K miles and still works and is quiet on all speeds.
Something I always do is never turn the fan to zero, always at least to 1 because I suspect if the fan sits idle for long periods of time, the commutator oxidizes which will cause noise when you do use it.
I noticed that after my car sat for about a month when I did the clutch, the fan was slightly noisy for a little while after I got the car back on the road.
Of course assuming the fan will last longer if it doesn't sit idle is just a theory on my part.