I've been annoyed for a long time with a high pitch noise problem when laptop not plugged to AC (i.e on battery). To my great relief the problem had gone with kernel 2.6.22 ... And now it's back with kernel 2.6.23 !!!
This is probably due to fan speed handling, I don't know really. Been playing around with kpowersave with no result.
Of course, if you plug the computer back to AC the noise stops immediately.
Please help, this is just driving me mad !!