Reciting Bismillah In The Heart
Saying Bismillah is not a compulsory requirement for Wuzu, but it was uswa of Prophet (sws) to say Bismillah before starting any good act, including wuzu. Since toilet is a not a clean place, therefore scholars advice to not say it loudly considering the sanctity of Allah’s name. This is again a matter of etiquettes, and one may say it in a low voice or in one’s heart as well. Even if you utter some sound while doing it, that is fine too.