We must knock before entering a room. The m'eel, one of the eight garments of the High Priest, was decorated with bells. Whenever the High Priest would enter the Bais Hamikdash, his presence would be announced by the jingling of the bells on his garment. Rabbi Yochanan learned from this the practice of always knocking on the door of his house before entering. This is one of the seven directives that Rabbi Akiva gave to his son Rabbi Yehoshua. "Don't enter your own house suddenly (that is, without knocking); all the more so, the house of your neighbor." Pesochim 112a, and Rashbam).
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