Толковый словарь английского языка Oxford English Reference - hum

1. v. & n. --v. (hummed, humming) 1 intr. make a low steady continuous sound like that of a bee. 2 tr. (also absol.) sing (a wordless tune) with closed lips. 3 intr. utter a slight inarticulate sound. 4 intr. colloq. be in an active state (really made things hum). 5 intr. Brit. colloq. smell unpleasantly. --n. 1 a humming sound. 2 an unwanted low-frequency noise caused by variation of electric current, usu. the alternating frequency of the mains, in an amplifier etc. 3 Brit. colloq. a bad smell. Phrases and idioms hum and haw (or ha) hesitate, esp. in speaking. Derivatives hummable adj. hummer n. Etymology: ME, imit. 2. int. expressing hesitation or dissent. Etymology: imit.