You should be able to adjust CPU throttling (make it more aggressive and therefore quieter) using the command-line utility pmset.
I'm on Windows and don't have my Mac handy at the moment so I can't give more detail.
Try man pmset
for more info on the command and it's parameters.
EDIT
I never got around to checking my answer and, as Václav Slavik pointed out to me, I should have because you can control most powersave settings with pmset but NOT the CPU throttling.
It seems the only way to do this is with messing with the plist files of the kernel extension that controls the CPU throttling.
That is tricky business and not something to average user should ever try to do.
For those that want to try anyway: Look for "CPU throttling" on the Apple StackExchange Site and will be several questions and answers that can point you in the right direction.