Your audience is chattering away amongst themselves because you've given them permission to become active, in some step that is designed to help cement your message into their minds.  It's chaotic, but the audience is happy, and cementing, and those are good things.  You're tempted to leave now, but you know you have to finish the speech off in some style.  And anyway, you're the one they came to see.   

Here's where you need to expend some energy to get them back.  They will come back, but you have to insist, politely and firmly, that they do.  You're asking them to sit down and become passive again, and that's asking a lot, so don't hold them for long and don't try to do much.

Your goal at this point is to remind the audience, in a stirring and powerful way, of the central theme of the talk.  Great closes are inspirational, and aspirational.  Remind the audience of the big reason they're all there, or point the way up the path to greatness, or quote some great words by some other orator if none will come to you. 

Keep it short (under 3 minutes, closer to 1 minute is better), keep it inspirational, and then NEVER FORGET to say 'thank you'.  That's the universally understood signal that a speech is done and the audience should applaud.  If you've done well, they'll leap to their feet. 

By the way, if you're going to take questions, then save this ending segment until the very end.  If you end with Q and A, then you're at the mercy of the last question.  Often, the last question is not the best one, and it may even be asked by a crank who has been waiting, and fuming, for some time.  So deal with the questions, but then close with your own statement.  Audiences tend to remember best the last thing they hear, so make it yours.

Next time I'll talk about thematic ways to think about the whole speech.