I have a 1999 A6 2.8 Quattro and I live in Florida, that requires the A/C to be ON a lot.
The electric cooling fan, most of the time when is ON make a very annoying noise, like a turbine, some time it goes down like in the evenings, I noticed few other Audis with the same problem.
Few mechanic told me is nothing wrong with the fan, it could be the it has being move and therefore the noise, few month ago I did the timing belt, and when they put everything together it seam to be less noisy, in fact I got so paranoid that if it did not made a noise I would open the hood and check if the fan was running.
Any idea or experience with this kind of problem, no guesses please, I have already went through the old "you should get a new car bullshit"

Thanks
Daniel

 

The electirc fans are pretty noisy. If you think its noisier than when it was new you may have a bad bearing in the fan. You can try replacing the fan motor.