Hard Drive Noise

[Jim Herrmann]

My wife's desktop is making an odd rhythmic sound that sounds like a 
quiet chain saw revving over and over again.  It does not continue 
without end, but runs quite a bit.  I believe it to be a disk drive 
making the sound, but it's hard to tell which one of the three drives in 
the machine is making the sound.  I have installed and run the smartmon 
utilities and they don't indicate any hard drive failure being 
imminent.  Any suggestions as to how I might figure out which drive is 
making the sound, and thus is the one headed south?

Thanks in advance for any suggestions.

Peace,
Jim