If musical instruments are forbidden on shabbat why do we adorn the crown of a (ashkenazik) sefer torah with bells?
don't know how much data mining counts as an independent answer but here is what I have found (the links include the textual citations)
If one has bells that chime whenever one opens a door and neglected to remove them before Shabbat, then the custom is to allow one to enter the house despite the knowledge that he will thereby produce the problematic sound. This is based on the Magen Avraham (338:1 and 301:35) who says that one can move curtains or clothes with little bells attached to them if he does not have intention to make the noise. The Mishna Berura 338:6 (see also Biur Halacha ad loc.) explains this opinion and allows following it in a case of need, for example, if it is the only way into his house. In the case of bells on the adornment of a sefer Torah, there are authorities who are lenient because of the mitzva involved (see Mishna Berura, ibid.), and each shul should follow its minhag and the ruling of its rabbi. ------------emphasis mine
this reviews a variety of opinions rangibg from "One who lifts and carries such a Sefer Torah creates a ringing sound, and it would therefore appear that just as one may not walk about with a bell on Shabbat, so is it forbidden to carry a Torah with "Rimonim." Indeed, the Taz (commentary to the Shulchan Aruch by Rabbi David Halevi, Poland, 1586-1667), in Yoreh Dei'a (282), writes that a Sefer Torah with "Rimonim" should not be used on Shabbat, due to this concern."
"The Eliyahu Rabba (work of Halacha by Rabbi Eliyahu Shapiro of Prague, 1660-1712) and others contend that this prohibition does not apply to situations involving a Mitzva. The "Rimonim" serve to adorn the Torah scroll and to announce the removal of the Torah from the Heichal, and the prohibition against making sounds on Shabbat therefore does not apply to the sounds produced by the "Rimonim." "