So I took the car back to the mechanic. RATTLERATTLERATTLE ... Halfway there the damn rattle stopped and by the time I got the workshop the car was purring like a kitten.
I left it there anyway, maybe when it cools it will do the rattle again.
Five minutes later they phone me and call me back. The bit of the radiator that the top hose connects to "has broken". Now, true, the plastic was rather brittle (plastic? Subaru, WTF?) but it wasn't broken when I left it with them. It now needs a new radiator tank, or possibly a whole new radiator.
So now I'm looking for a new mechanic.