My 2000 s4 is leaking coolant from somewhere behind the engine but I can't see where it's coming from. Is there a common part that begins to leak back there? The reservoir will empty sometimes in as quickly as a few hours, other times it will take days.

 

I had the what sounds like the same leak you do and it turned out to be my auxillary water pump. I'd say check that out.

 

Where is that located and how hard is it to change?

 

It's not fun but here is a write up i found. I hope this helps good luck.

http://forums.audiworld.com/s4/msgs/1585786.phtml