Perhaps, it is your limited knowledge of British Magical History that is hindering you from correctly guessing the artefact. Although kudos to you for not giving up so easily, Ilvermornian!

As you take a look inside the piano once more, a huge rat took its chance to emerge from the side and ran right under the bed. And on its wake, you caught sight of a dark cloth in between the rusty piano strings.

Ah. Perhaps it is the right artefact? If you noticed earlier, the last sound of the key you pressed did sound muffled.

A house it does choose for your Hogwartian friends?