Did a quick design. Search for '555 Wizard', you should find a bunch.
Usually, I mess around until I find standard values. You can use trim pots to fine tune, or experiment with higher/lower frequencies.
This is an old thread (last summer). I thought it was interesting, and some of the comments and opinions amusing. Anyway, I built something similar a few years ago to scare off cats. The circuit posted is just for a constant 18k 50% duty cycle square wave. My has 2 timers and only gives short bursts ever 10 seconds or so. 555 timers are very temperature sensitive, and drift quite a bit. Mine ranges from just a click from the speaker (actually over 20k signal), to an audible tone. Not very effective on cats, but leave it running anyway.
The output isn't too bad at 12 volts, but you might want to amplify it some.
It's only a minor annoyance, might not bother anyone enough to make much difference, as with the cats. Doubt it's going to damage anyone's hearing, or be a huge problem for the neighbors. The sound doesn't carry that far.