Which word defines noise that is loud, but less than a din?

Solution 1:

There is 'clamour' (US: 'clamor')

Clamour - noun

1 - a loud uproar, as from a crowd of people: the clamor of the crowd at the gates.

4 - any loud and continued noise: the clamor of traffic; the clamor of birds and animals in the zoo.

www.dictionary.com

There is also 'hubbub'

Hubbub - noun

1 - a loud, confused noise, as of many voices: There was quite a hubbub in the auditorium after the announcement

www.dictionary.com

Solution 2:

Agree with Marv that clamor and hubbub are nice ones for the sustained noise of voices in a room.

You might also consider racket, which I think suggests a more abrasive sound than either of the above, perhaps one punctuated by the tapping of journalists fingers on keyboards.

http://www.merriam-webster.com/dictionary/racket

noun

1 : confused clattering noise : clamor

2 a : social whirl or excitement

Solution 3:

Although Oxford Dictionary’s entry for “buzz” includes the notion of “continuous”:

1A low, continuous humming or murmuring sound, made by or similar to that made by an insect: the buzz of the bees a buzz of conversation;

I usually use, perhaps redundantly, and see it with “constant” (which I suppose makes this a two-word answer), as in this example from ‘The Smouldering Flame’ by Anne Mather (via Google Books):

… she doubted if he had even heard her above the constant buzz of conversation all around them.”