Make darn sure that the HD themselves are secure in their mounting and/or location. Rattle from metal contact can cause disturbing sounds but is normal unless actually unsecured. Next, some sound should be present provided it gets accessed alot and the location of data as it physically seeks it all, "IF" appears to increase during intense operations. Last, what maybe considered a HD noise maybe more about some physical fan as the sound source. I would think any SDA would be a quiet operation so be sure what is causing the noise or sound source excluding those.
You didn't provide full details on the PC this is configure on, so give.
Ubuntu-12.04.3 on sda
Mageia-3 on sdb
Mbr on sda grub 1.99
From time to time I get a chatting sound from one of the hard drives
Ran smartmontools and both drives passed
Is there a way to locate which hard drive is causing the noise