I used to have that issue on my white macbook... my macbook pro (june 09) is quiet (with headphones)
If you're hooking up to an amp or any type of audio gear you should probably be running a external audio interface anyway... I fixed my noise issue with a $40 dolar behringer interface on my white macbook
I use an external m-audio cheapy weepy on the mbp does the trick for gigs etc...