What you said
Silence is difficult to maintain. When I was teaching, I would say "quieter" several times, reducing my own volume until the pupils had reduced theirs sufficiently. No unrealistic target, and everyone understood their role and responsibility.
With a class like that, I stand at the front watching them, so as to see who are the miscreants. Warn them that, having asked nicely, you will now be giving punishments to all who talk out of turn. If you're consistent, and if the sanction is serious enough, they'll eventually get the message and shut up.
The expert view
Hummers? Bummer. This situation sounds like a very normal problem: most children want to work, while some want to gambol and caper.
One thing: they are deliberately trying to wind you up. Children often see teachers as sport and it seems that some of them do here. They have heard your rules; they have seen you annoyed; they have probably enjoyed it and had a good laugh later on.
Back to basics is good. Tell them that most of them are doing really well and you are proud of them. That way, they will not think you are just some grumpy tyrant. Then remind them of your expectations and consequences, so they can see that you are initiating a change. Then make it happen.
Set tasks during which they work in silence. Then simply make a list of names of anyone who breaks the silence. Punish them after school with half an hour or so of writing out the class rules, or lines, or anything that makes them unhappy.
The point is to make them not want to be there. No hugs, no reconciliation.
At the end, you can remind them that you want them to do really well but that they are impeding their own progress. Then: repeat, repeat, repeat.
Hummers are the worst. There should be a special level in Dante's Inferno for them. If you are repeatedly saying "That's not silence", then they are not learning not to do it. You have to go beyond the verbal reprimand, which for many is no reprimand at all.
Keep this up. They will resist you at first and perhaps get worse for a while. But eventually they will realise that you are serious about what you say.
No shouting, no snarling - just simple instructions and actions, repeated until you grind them down into sociability.
Tom Bennett is author of The Behaviour Guru and Not Quite a Teacher. Read more from Tom on his blog, behaviourguru.blogspot.com, or follow him on Twitter at @tombennett71. His latest book, Teacher, is out now, published by Continuum. Post your questions at www.tes.co.ukbehaviour.