.

[tidy]

.

 

[Elixer]

Could be SMART checks.

 

[VirtualLarry]

head wear leveling, background surface scan checks

 

[bononos]

........
For example, my WD AAKS drives (older black versions) eventually start making a continuous tick...tick...tick (~four seconds apart) when they're not being accessed (and the sound ceases the second they are accessed). If I listn really closely I can hear modern WD drives doing something similar, although it's much quieter (in keeping with the newer drives' quieter seek noise compared to the AAKS).

Another example is an old 80GB barracuda of mine, a model known for it's noisy seek, will often launch into a cacophony of frantic activity when completely idle. BuzzzzCLICKzzzzzzCLICKzzzzzzCLICKzzzzzzCLICKzzzzz.... Absolutely nothing to do with access, because it happens even when the drive is running with no data connection.

So what exactly are drives doing so busily when they're 'not doing anything'? Am I right in assuming it's some kind of wear levelling or other housekeeping?

Head wear levelling would make sounds at fixed intervals so you can check the timing on the sounds you are noticing. 4secs sounds a little too quick. Maybe you could email support and let us know the head levelling interval.