When I got my x61 laptop, I did not remember that it made this high pitch sound it now does. I believe that it came along with one of the Ubuntu Hardy kernel upgrades. The sound is only present when the machine is on battery power, and it was driving me crazy!
I Googled around, and found other referring to the same noise, though on other types of laptops. The problem seems to be, when max_cstate is at level C3 or higher. So I put it on C2.
You have to do this in the /etc/modprobe.d/options file, since you have to make a new initrd image, because this module is loaded from initrd at boot time.
Add options processor max_cstate=2 to /etc/modprobe.d/options and then update the initrd image (update-initramfs -u).
After you have rebooted, you can check the max_cstate in /proc/acpi/processor/CPU0/power too see that you are at the desired level.
And listen… no noise! 😀