Not to be negative, but I thought I would explore the possibility of there being a problem rather than a normal sound/function.

The raid could be rebuilding itself if there is a problem with one of the drives, or even the controller hence more utilisation.

The mirror you explained is for redundancy - if one drive fails you can still boot your computer with the good drive and even take it out of the raid if you do not want to replace a faulty drive. Typically if you break the raid and later try to rebuild it will wipe all disks (initialise) - but not always.

If one disk is failing, running tools will help identify it but you may need to disconnect one drive and run the tools and then swap the HDDs over and run the tool again. Some HDD diagnostics will not be able to distinguish between disks in a RAID.

Alternatively, and indexing service will increase disk access. And one other thought is that the noise may be coming from the case's cage holding the drive - I always liked the cages that had rubber washers between the drive and cage absorbing some vibration. I have seem some really cheap white-box cases can generate their own noise.

.... I am sure things are fine, after all your data is protected with the mirror but I would certainly investigate the idea of a faulty mirror/drive.

CTO