I dont know how it started, or how it got that loud. But recently it reached an alarming peak and was driving me mad in a quiet room. The internet will tell you that loads of Mac users have this issue which I think is related to battery and power supply. It doesnt affect the performance of the computer at all. But it is fucking annoying.
And then I accidentally found a simple solution: Simply run Coconutbattery.
By running coconutbattery the whine instantly goes away. and you get a battery meter all the time. its freeware and its small. i'm using coconutbattery 2.5.1 which isn't the latest one but hey it does the job and i aint about to try to fix it if it works. there must be other solutions to cutting down the macbook whine (which happens when the disk is idle) but this is my weird solution. There's a program on the internet for reducing Macbook pro whine, but it probably works for Macbooks too.
I am using an old macbook which is, according to coconutbattery, now 31 months old. thats almost 3 years now!