May one whistle [or make other musical sounds] with one’s mouth on Shabbos?
It is Rabbinically forbidden to play musical instruments, or make music using the movements of one’s body, on Shabbos. Nevertheless, the Sages limited this prohibition only to the doing of an action [with a bodily limb, such as one’s hands or feet] to sound music, however, it remains permitted to sound music using one’s mouth. Thus, it is permitted to sing on Shabbos. Furthermore, it is permitted to sound music using one’s mouth even if it is not in the form of singing, but is rather similar to the sound of an instrument or the sound of a bird, such as those who call at their friend and whistle like a bird. This allowance applies even if the whistling sound is in the form of a tune. [Thus, it is permitted to whistle on Shabbos for any purpose and to any tune. Likewise, one may sound other forms of musical notes with one’s mouth in imitation of a musical instrument, as is commonly done in a Capella. Indeed, based on this, it was an old Ashkenazi custom to sound music using their mouths and make sounds identical to an actual musical instrument, This was done even by the holy Rabbis and Chassidim of Ashkenaz. The Rebbe was accustomed to telling people to whistle on Shabbos during the Niggunim of the Shabbos gatherings.]
It is permitted to whistle on Shabbos without restriction.
May one whistle through placing his fingers in his mouth?
Some Poskim rule it is permitted to whistle even through placing one’s fingers in his mouth. However, other Poskim negate this allowance, and so is the simple implication of Admur. Practically, one may be lenient to whistle a single sound, and not a tune, with the help of his fingers. [As stated above, the Rebbe would motion to people to whistle on Shabbos with their fingers, although seemingly this was done as a single sound and not a tune. Furthermore, making sounds for the sake of Simcha Shel Mitzvah is customarily permitted even when using the body, and hence no further leniency should be derived from it.]
May one whistle with the help of an instrument, such as a whistle, or fork or spoon?
 See Admur 338:1-2; Aruch Hashulchan 338:6-8; Piskeiy Teshuvos 338:4
 Admur 339:2; 338:1 “However, not with another matter”; Michaber 339:3; Mishneh Beitza 36b
 Admur ibid; Michaber 338:1; Rambam Shabbos 23:4; Raba Eiruvin 104a
The reason: This is forbidden due to a Rabbinical decree that one may come to fix a musical instrument. [Admur ibid]
 Admur 338:2; 338:1 “It is forbidden to make sounds of music, with exception to one’s mouth”; Rama 238:1; Darkei Moshe 338:4; M”A 338:2; Shiltei Hagiborim Eiruvin 35b in name of Riaz; Aruch Hashulchan 338:6-8
The reason: As the decree against sounding music is due to worry that one may come to make a musical instrument, and this worry is only applicable when one does an action [with an item or bodily limb] to make musical notes, as then there is room to suspect one may come to fix an instrument. However, when sounding music merely with one’s mouth, there is no need to worry that it may lead to fixing instruments. [Admur ibid; See Levush 338]
 Admur 338:2; Rama 238:1; Darkei Moshe 338:4; M”A 338:2; Shiltei Hagiborim Eiruvin 35b in name of Riaz; Aruch Hashulchan 338:6-8 [See there that he questions this ruling of Rama, as being obvious and superfluous, as all mouth sounds are permitted, thus concluding that the novelty here is that one may whistle even with his fingers in his mouth]
 Admur ibid; M”A ibid [See Machatzis Hashekel 338:2]; P”M 338 A”A 2; Machazik Bracha 338:1 that so is the custom and ruling of Achronim; Aruch Hashulchan 338:6-7 that so is Pashut!; M”B 338:3; Orchos Chaim 338:4; See Kaf Hachaim 338:7; Piskeiy Teshuvos 338:4; Rav Yaakov Yosef and his father Rav Ovadia Yosef rule it is permitted
Other opinions: Some Poskim rule that it is forbidden to whistle to a tune on Shabbos, and only whistling for the sake of getting someone’s attention is permitted. [M”A ibid in implication of Shiltei Hagiborim and Darkei Moshe ibid that it is only permitted if a) an action is not done and b) a musical tune is not sounded; Machazik Bracha 338:1 and Birkeiy Yosef Shiyurei Bracha 338:1 in name of Givat Pinchas 1:36 that it is forbidden to whistle like a bird; Implication of Makor Chaim 338 [Chavos Yair]; See Kaf Hachaim 338:7]
 Machazik Bracha 338:1 “On this the Ashkenazim rely to sound music using their mouths and make sounds identical to an actual musical instrument, and to the bystander it actually sounds like musical instruments are being played. I heard this is done even by their holy Rabbis and Chassidim, and so is the arbitration of the Achronim, that it may be done”; Kaf Hachaim 338:7; See Aruch Hashulchan 338:6-8
Other opinions: Some Poskim rule that it is forbidden to make musical sounds with one’s mouth on Shabbos. [Machazik Bracha 338:1 and Birkeiy Yosef Shiyurei Bracha 338:1 in name of Givat Shaul 1:36]
 Machazik Bracha ibid; Kaf Hachaim 338:7
 See Hiskashrus 674
 Aruch Hashulchan 338:7; See there that he questions the ruling of Rama who permits whistling, as being obvious and superfluous, as all mouth sounds are permitted, and he thus concludes that the novelty here is that one may whistle even with his fingers in his mouth;
 See Mishneh Halachos 4:52 who negates the reasoning of the Aruch Hashulchan; Rav Yaakov Yosef in name of his father Rav Ovadia Yosef rules that one should not do so [in retraction of Rav Ovadias ruling in Yabia Omer in name of Aruch Hashulchan ibid]
 It is implied from Admur and Poskim ibid that using one’s hands is forbidden due to it being an action, as what difference does it make if one claps his hands or uses his fingers
 See Piskeiy Teshuvos 338:4
 See Hiskashrus 674
 Aruch Hashulchan 338:8; Piskeiy Teshuvos 338:4