| A battery, a 555 timer chip used as an oscillator, and an 8 ohm speaker would be the simplest way to go. (You'll need some resisters and some coupling capacitors, too.) If you add a second 555 chip as a timer, you can adjust the amount of time the first one will buzz for.
As for circuits that will do the job, make a web search for "555 timers" on Google or any other search engine you prefer. You'll get more circuits than you need.
If you've never worked with ICs before, find a tutorial on 555 timers. There are plenty of those around, too. As chips go, they're relatively easy to understand.
Good luck. | 
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