Question: Can one use a doorknocker on Shabbos?
Answer: The Gemara (Beitza 36b) writes that chazal forbade one from dancing and playing with musical instruments on Shabbos as they were worried that they may come to fix a broken instrument (makeh bepatish).
Thus, the Gemara (Eruvin 104a) discusses whether one may make sounds that have no melody such as knocking on a door. Based on this, Rambam (Shabbos 23:4) and the Beis Yosef (OC 338:1) write that one mustn’t use any instrument that makes noise as we are concerned that one will use it to play music. The Mishna Berura (338:1) writes that the same applies to objects that aren’t created for noise. Thus, one should even avoid clapping in the normal way.
The Biur Halacha (338:1) quotes Rambam (Pirush Hamishnayos, Eruvin 104a) who implies that one may use a door knocker on Shabbos. Nonetheless, the Rema (OC 338:1) writes that one must only knock on a door on Shabbos with one’s hand and not with a door knocker.
R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 28:35) writes that one may use something else, such as a key, to bang on the door, providing that one doesn’t do so in a specific rhythm.In conclusion, one shouldn’t use a door knocker on Shabbos nor should one knock in a specific rhythm.