I've changed two tandem pumps. Both within two weeks of eachother. One on a Passat 2.0TDi and the other an Audi A3 2.0TDi.
One engine had a rattle like yours, the other was rattling and dumping oil into the fuel.

We bought cheap ones from ebay and they're still going well 2 years later!
Not a hard job at all. Hour at most!