I'm familiar with most hard drive sounds. When I first heard this I replaced the drives in my mirror, and I still have it. Could it be something that the RAID controller is making the drives do? (Intel controller on motherboard) It doesn't sound like something catestrophic, but it is annoying, and I can't help from think that it could be related to some system sluggishness.
(I'll try to attach something, but I only had a few minutes and my phone camcorder to capture something, and it might be too large to upload. Also not sure if the recording will faithfully reproduce the low freq buzz. )