Hej Argenta!
Well it might be what your indy diagnosed, the water pump.
A failing bearing for the water pump or the other tensioners and pullies along the serpentine belt, can make all kind of rattling and knocking sounds that may come and go.
Here are some examples:
https://www.youtube.com/watch?v=A5F9ATnv04A
https://www.youtube.com/watch?v=4bEgOXPRQvE
Ah, well if you changed oil it's quite possible that something that was a bit sticky/under-lubricated freed up (although you'll probably never know what it was). Glad it's sorted, either way.