My refrigerator is about 15 years old and...

The compressor is sitting on springs inside a metal case. When it stops, the centrifical force is absorbed by these springs. If one or more springs become weak or break, the compressor hits against the outer case a time or two. (CLUNK CLUNK clunk) There is really nothing you can do about it, short of replacing the compressor. (read big bucks) NOTE: If you take a new compressor and give it a shake, you will hear the same clunk. If the noise is really bothering you, I'd say after 15 years, it's time to move that refrig to the basement and get a new energy star model for your kitchen. (the old one most likely has several more years worth of clunking before it truely dies)