Mac users: is there any way to get rid of the noise when you turn the computer on....that "bonggg" noise?

Yes, but it can be complicated. You should keep the noise, the computer does a quick check of itself and if you hear the "bonggg' noise everything is good. Glass braking or chimes means something is wrong.

Bruce good answer gave you a thumbs up, when I do get tired of the sound and forget to turn off the volume as I don't want to wake anyone I use my iPod or any ear-buds and put into the "Earphone Port"

DaDa it works every time I know of a Mac User David Pogue and his trick when on an airplane is he carries just the "Jack" no cable just the Jack and puts it in the port.

Mute the computer before you turn it off.