hard drive - HDD constantly making a power-up noise

I have two hard drives installed on my system - WD Caviar Black 500GB (primary one) and a Samsung Spinpoint F4 320GB (used for storage). The first one has been in use for a year and the second one for four years.
The problem I am experiencing is a peculiar one, often when I (or a piece of software, doesn't matter) have to read data from the disk, it makes a power-up or a spin-up noise (a long buzz followed by a click), but not always.
This sound is perfectly normal when turning the PC on or off but not in between. I also don't know which one of the two drives is actually making the noise but I'd guess it is the primary one.
Hard Disk Sentinel says that both of them are in perfect shape (no weak sectors or anything like that) and that they pass the S.M.A.R.T test - status "OK" on all of the values and "Always passing" on most of them.

This has been going for about six months, I believe, but there have been no serious problems with either of the drives even though I suspect that one of them is dying.

Cannot upload pictures so I am posting links instead.

HD Sentinel - http://i.imgur.com/3VGDRcV.png (Samsung's Raw Read Error Rate is weird)

Speccy snapshot (hope it's not too much bother): http://www.speedyshare.com/mwMWn/system-specs.speccy

Additionally, the PSU I have is a Corsair TX750.

I will, of course, provide more information if needed. Thank you all in advance.

Answer

If you have the drives set to power down when not used, it's "normal" to hear regular "spin up" sounds when you access a drive after some inactivity (and this may also occur occasionally when background tasks access the drive).

Depending on your OS version and the vendor-supplied power management software there may be a way to turn on and off and adjust the timeout for the power down of inactive drives.

However, if you hear a drive repeatedly "spinning up" again and again over periods of a minute or three it usually means that something is wrong with the drive (it's failing to start up), and quite probably you will notice that your computer is "hung" while this occurs.