hum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To emit a continuous low droning sound like that of the speech sound (m) when prolonged.
• intransitive verb To emit the continuous droning sound of a bee on the wing; buzz.
• intransitive verb To give forth a low continuous drone blended of many sounds.
• intransitive verb To be in a state of busy activity.
• intransitive verb To produce a tune without opening the lips or forming words.
• intransitive verb To sing (a tune) without opening the lips or forming words.
• intransitive verb Baseball To throw or pitch (a ball) very fast.
• noun The sound produced by humming.
• noun The act of humming.
• interjection Used to indicate hesitation, surprise, or displeasure.

from The Century Dictionary.

• An interjectional, hesitating sound, uttered with or during a pause; hem; h'm.
• noun Any inarticulate, low, murmuring, or buzzing sound, as that made by bees in flight, by a spinning top, etc.; a faint continuous sound having no definite pitch; a buzz.
• noun Specifically— A low confused noise, as of a crowd, or of distant voices or sounds of any kind.
• noun A buzz or murmur of applause or approbation.
• noun A sound uttered with closed mouth by a speaker in a pause from embarrassment, affectation, or the like: as, hums and haws. Also (and now more commonly) hem.
• noun A drink formerly common, probably made of strong ale or of ale and spirit. Its exact composition is not known.
• To make a prolonged droning sound, as a bee in flight; drone; murmur; buzz.
• To give utterance to a similar sound, such as the droning of a tune, a contemptuous or vacant mumbling, a murmuring expression of applause or satisfaction, hesitation, dissent, etc.
• Same as hem.
• To sing with shut mouth, as to the sound m; murmur without articulation; mumble: as, to hum an air.
• To express approbation of, or applaud, by humming.
• To trick or delude; impose on; cajole.
• noun An imposition or hoax; a humbug.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To make a low, prolonged sound, like that of a bee in flight; to drone; to murmur; to buzz.
• intransitive verb To make a nasal sound, like that of the letter m prolonged, without opening the mouth, or articulating; to mumble in monotonous undertone; to drone.
• intransitive verb To make an inarticulate sound, like h'm, through the nose in the process of speaking, from embarrassment or a affectation; to hem.
• intransitive verb To express satisfaction by a humming noise.
• intransitive verb To have the sensation of a humming noise.
• transitive verb To sing with shut mouth; to murmur without articulation; to mumble.
• transitive verb To express satisfaction with by humming.
• transitive verb Colloq. & Low To flatter by approving; to cajole; to impose on; to humbug.
• noun A low monotonous noise, as of bees in flight, of a swiftly revolving top, of a wheel, or the like; a drone; a buzz.
• noun Any inarticulate and buzzing sound.
• noun The confused noise of a crowd or of machinery, etc., heard at a distance.
• noun A buzz or murmur, as of approbation.
• noun An imposition or hoax.
• noun An inarticulate nasal sound or murmur, like h'm, uttered by a speaker in pause from embarrassment, affectation, etc.
• noun obsolete A kind of strong drink formerly used.
• noun See under Venous.
• interjection Ahem; hem; an inarticulate sound uttered in a pause of speech implying doubt and deliberation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A hummed tune, i.e. created orally with lips closed.
• noun An often indistinct sound resembling human humming.
• noun Busy activity, like a beehive's buzz
• noun UK, slang unpleasant odour.
• verb intransitive To make a sound from the vocal chords without pronouncing any real words, with one's lips closed.
• verb transitive To express of affect by humming
• verb intransitive To drone like certain insects naturally do in motion, or sounding similarly
• verb intransitive To buzz, be busily active like a beehive
• verb intransitive To produce low sounds which blend continuously