After credits, on "The end" screen, you can hear sword

[Renkineko]

If you use the attack button while on the screen of "The end", you can hear the sword of the hero (and you even can charge it and do a special move ^^). Didn't try the other buttons, but I suppose you can hear them too depending on the last item you had equiped.

Not very disturbing, just fun. You can even let it so it's a feature =P