May one use a door knocker on Shabbos?
A. Background-Making noise with an item on Shabbos:
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 tone of music with this noise.
B. The Law-Using a door knocker:
One is not to use a door knocker to knock on a door on Shabbos, due to opinions which forbid its use. Rather, the custom is to knock with one’s hands on the door. Some knock with an instrument that is not designated for knocking. [Thus, one may knock using a key, a book, or any item that is not designated for knocking.]
 Admur 338:1
 This refers to random sounds which have no rhythm or beat such as a simple knocking on a door.
 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. [ibid]
 Admur brings a dispute regarding this matter: The first opinion [Michaber; brought as the Stam opinion] rules 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 opinions [Rama 338:1] however rule one may not make any sound, even a mere noise, using an instrument which is designated to make noise. Practically the custom is like the latter opinion. [ibid]
 Admur brings a dispute regarding this matter: The first opinion [brought as the Stam opinion] rules that it is permitted to knock on a door using its knocker. Other opinions however rule one may not do so being it is an instrument which is designated to make noise. Practically the custom is like the latter opinion. [ibid] Vetzaruch Iyun if this ruling is based on Admur’s legal conclusion or based on custom. Tzaruch Iyun if when Admur wrote the custom he was intending to give a final ruling like whom we follow and hence we follow the stringent opinion based on custom, or perhaps previously when he wrote “it is forbidden to knock using the door knocker” this was the final ruling, and when he added the custom to knock with one’s hand, it was to exclude knocking with an undesignated vessel.