Is hearing a woman humming considered kol isha? Can you site the source please? Thanks.
There are various indications that humming a slight tune is not included in the prohibition of Kol Isha, hearing a woman sing. However, a person must distance himself from anything which may cause him improper thoughts.
See Magen Avraham to Siman 689 who rules based on the Talmud, Megilla 23a that a woman should not read from the Torah or Megilla for a crowd [which includes men] only because this is considered disrespectful, but there is no concern of Kol Isha [cf. Orchos Chaim Hilchos Megilla 2]. See also Mishna Moed Katan 3-8 that it wa customary to bring women to wail and chant at funeral services, this is codified in Shulchan Aruch Y:D 344:3 [cf. Tiferes Yisrael to mishna ibid.]. See Shaar Hatziyon 560:25 that a woman should not sing her child to sleep in the presence of other men, in the Sefer Om Ani Choma [Gross, 21:1, an authoritative work on these matters] he explains that this refers to actual singing, but slight humming as is customary would be permissible.