It could just be your HDD accessing data but it does sound a little ominous.
You could try disconnecting whichever drive is the slave in order to identify which drive the noise is coming from.
I personally would consider backing up any critical data once you know which drive is making a noise as it could possibly be a HDD failure pending.
One of my scaleo machines made an odd noise from the HDD a few weeks ago and a few days later it was goodnight Vienna, and I did not back up the data from that one, big headache.
Hope this is not the case for you
Learning all the time.