I have a 97 Audi A6 wagon. It just started making this hideous noise in neutral (sometimes in park), but not in D or R.
What hideous noise? I’m having a hard time hearing it from here. ;-))
But seriously, it is rather difficult to give you a good idea of what is going on, given the paucity of information that you have provided. Despite that, I will venture to say that you might simply have a loose heat shield.
Of course, that is just a guess. Give us more complete information, and we just might be able to give you an answer with more substance.