Is there any source that whistling in a room summons demons?

No. It’s a superstition without any source.[1] What is true that it is recorded in Sefarim^[2] that whistling has been traditionally included in some witchcraft ceremonies, however not that all whistling is considered witchcraft or that all whistling can summon spirits and demons. To put the matter to rest, it is an explicit ruling in the Rishonim and Poskim^[3] that whistling is permitted, and that it is permitted to whistle even on Shabbos.

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[1] See Shabbos Beshabato p. 327

[2] Eiyn Zocher of Chida Mareches Alef 34; Koach Shur 31

[3] Admur 338:2; Rama 238:1; Darkei Moshe 338:4; M”A 338:2; Shiltei Hagiborim Eiruvin 35b in name of Riaz; P”M 338 A”A 2; Machazik Bracha 338:1 that so is the custom and ruling of Achronim; Aruch Hashulchan 338:6-7; M”B 338:3; Orchos Chaim 338:4; Kaf Hachaim 338:7; Piskeiy Teshuvos 338:4; Rav Yaakov Yosef and his father Rav Ovadia Yosef rule it is permitted ; See Hiskashrus 674; Sefer Hasichos 5752 p. 186;