yup. easier than changing it, is accepting it.
learn to love that sound as the sound of a healthy mac. if you turn it off, and your computer happens to go tits up, you will have lost the first port of diagnosis. if it's a problem that it might wake someone, keep a set of headphones near the mac, which you can plug in if you reboot at night, the sound will get bypassed into the headphones. to be honest, even just a minijack cut of the end of a set of headphones would work, as a totally effective and removable 'patch'.