I have a 2001 Audi s4. When the ac runs a fan rattles. I can get it to stop by running it on econ. But then thec does not get cold enough. The noise is coming from the front underthehood by the radiator but not sure what to replace... any help is great. We try to do all the work by ourselves so direction is great too...

 

Sounds like your auxiliary cooling fan is coming apart.
Do not run the A/C with a bad cooling fan!