I know exactly what you mean, my Black Macbook does this as well. I have had mine for just about a year now, and the "sound" has not had any impact on the operation of performance of my machine.
I have just had to deal with the fact that my macbook in particular makes a plethora of noises.
I have the creaky hinge, the flexing creeking left side, buzzing processor and noisey hard-drive.
The hard-drive sound never worried me, mainly because it is so incredibly faint, and compared to the sounds that my external hard-drives make (WD and G-Drive) not to mention the sounds my HP notebook makes.
I have had the IT guys in my company take a look at it, and listen to the sound, and they are positive its not leading to a failure, BUT I still have a bootable back-up just in case.
Honestly, out of all the macs I have seen in my company (clothing design) all the MBs and MBPs have all been nosisy in some way or the other. I love my Mac to death, but I have come to accept the fact that it is far from perfect. I put up with all the above mentioned issues because it is still better then working on a silent windows machine.