The advice here to use leather washers is only partially correct. Leather washers should only be used where the fender meets the frame (eg seatsay bridge, chainstay bridge). Don't use them where the stay meets the fender or where the stay meets the dropouts. The masters (Alex Singer, Rene Herse) did it this way and it works. They would also fashion a thin flat curved washer that matched the curvature of the fender and fit that on the inside of the fender where it meets the frame. This helps to make that connection more rigid and spreads the load on the fender so that it's less likely to crack.

Properly installed aluminium fenders do not rattle. Jan Heine has published some pretty detailed articles in Bicycle Quarterly explaining how to mount them correctly. Peter Weigle also has a trick to reduce the amount of eyebolt that sticks out of the fender, also published in Bicycle Quarterly. It might be worth your while purchasing the back issues with the relevant articles.