May one use a door knocker on Shabbos?
Background-Making noise with an item on Shabbos:
The sages prohibited one from sounding music on Shabbos, due to the crazy one way come to fix musical instruments. Included in this prohibition is sounding any sound with the intent of making it into a tune or melody. However, if one’s intent is simply to make a noise and not a melody or tune, then the following law applies:
Item not designated for noise making: It is permitted to make noise on Shabbos using an item that is not designated for noise making, so long as one does not intent to sound a tune of music with this noise.
Item designated for noise making: [It is disputed amongst Poskim as to whether an item designated for noisemaking may be used to make noise on Shabbos. Some Poskim rule that it is permitted to make noise with any instrument if one does not intend to sound a tone, even if the instrument is designated for music.] Other Poskim, however, rule that it is forbidden to use an item which is designated for noise making to make noise on Shabbos, even though no tune or melody is being sounded, due to a decree that one may come to fix it. Practically, the [Ashkenazi] custom is like the latter opinion. [Furthermore, some Sephardim are also accustomed to be stringent like the latter opinion, although others are lenient like the former opinion.]
The Law-Using a door knocker:
Based on the final ruling, and custom stated above, one is not to use a door knocker to knock on a door on Shabbos even though one has no intent to sound a tune or melody. Accordingly, it is forbidden for the community Shamash to knock on doors to call people to come to Shul using the set door knocker. Rather, he is to knock on the door with one’s hands, and so is the custom. Some communities, however, are accustomed [not to use their hands to knock, but rather] to nock with another instrument that is not designated for knocking. [Thus, one may knock using a key, or any item that is not designated for knocking. The above however only applies for Ashkenazim, however amongst Sephardim, many are lenient to permit knocking on the door with a door knocker.]
Ashkenazim may not use a door knocker to knock on a door on Shabbos.
If one cannot enter the home without using the door knocker, such as nobody can hear him knocking with his hand, may he use it?
 Admur 338:1; See Piskeiy Teshuvos 338:1-2
 Admur ibid; Michaber 338:1; Rambam 23:4; Raba Eiruvin 104a
 Admur ibid; M”A 338:3 as explained in Machatzis Hashekel ibid; Raba Eiruvin ibid
Other opinions: Some Poskim rule it is forbidden to make any noise using a vessel, even if it is not designated for this purpose. [Gr”a; Rabbeinu Chananel brought in Tur 338; Yerushalmi Beitza 5:2; Orchos Rabbeinu 1:168]
 This refers to random sounds which have no rhythm or beat, such as a simple knocking on a door.
 Implication of Stam ruling in Admur ibid who writes Viyeish Omrim; Implication of Michaber 338:1, as implied from M”A 338:1 and as explains Machatzis Hashekel ibid in his first explanation that the M”A/Michaber he is arguing on Rama ibid; Beis Yosef 338 argues against Iggur; So also learn the following Poskim that the Michaber ibid [and consequently the first Stam opinion in Admur ibid] is arguing on the Rama: Biur Halacha 338:1 “Al Hadelet [explains that a) The Beis Yosef questions the ruling of Iggur, and that from Pirush Hamishnayos of Rambam it implies that it is permitted] “Hoil” in brackets that seemingly the Kalkeles Hashabbos learns that the Michaber ibid argues on Rama [and explains that the fact the Rama did not write Yeish Omrim does not necessarily mean that it is not under debate]; Implication of Aruch Hashulchan 338:2-4; Or Letziyon 2:39-1 concludes that the Michaber argues on Rama and hence permits it for Sephardim; Piskeiy Teshuvos 338:1-2; Rav Yaakov Yosef z”l; See Hagahos Maimanis on Rambam ibid; Ravaya 795; Tur in name of Rabbeinu Tam
 2nd opinion in Admur ibid; Rama 338:1; Iggur 518; Shut Maharil 38; Beis Yosef 338 in name of Iggur
 Meaning, even if it is not designated to make music, but mere noise, such as a metal door knocker, it is not to be used. [Admur ibid]
 Admur ibid; Levush 338:1; See Kaf Hachaim 338:9
Other reasons: Some Poskim Rule it is forbidden because we may intend to make music with it. [Beis Yosef 338; Olas Shabbos 338:1]
 Admur ibid; Rama ibid; Darkei Moshe 338:3; See Kitzur Shelah P. 86; Yifei Laleiv 338:2; Kaf Hachaim 338:10
Other opinions: Some Poskim imply like the former opinion even according to Ashkenazim. [See Aruch Hashulchan 338:2-5 and 9]
 Chida in Tov Ayin18:68; See Yifei Laleiv 338:2; Kaf Hachaim 338:10
 Or Letziyon 2:39-1
 Admur 338:1
 Admur ibid; Rama 338:1
 Admur ibid; M”A 338:3
 Or Letziyon 2:39-1 concludes that the Michaber argues on Rama and hence permits it for Sephardim; Rav Yaakov Yosef z”l
 Kalkeles Shabbos, brought Biur Halacha 338:1 “Hoil” and Kaf Hachaim 338:10
 See Biur Halacha ibid