My one question is where the ''bonging'' coming from...your computer or the speakers? Assuming it's the speakers, have you modified the sound schemes, installed a custom theme, etc? Try going start->control panel->(sounds, speech, and audio devices)->sounds and audio devices->sounds and see what scheme is listed, as well as what sound is set to play for ''start Windows.''
Another possibility is that a start-up program is making the sound. Thus, you might want to check out what programs are launching at boot (start->run->msconfig->services and startup tabs), then check the options for any that might be the possible cause. Finally, since a lot of such programs add their sound files to the control panel options, you might want to scan the list (not just under ''Windows'') and see if anything sticks out. (Select anything that looks/sounds like it may be it and press the 'play' button.)
Hope this helps,
Whenever I start Win XP Home, I get 12 bongs in fairly quick succession during startup. There are no messages that pop up and everything seems to operate as it should. Anyone have an idea what this is all about?