Hard Drive Noise

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