Monitor the S.M.A.R.T. status of your hard drives.
In the past several years I owned two hard drives which failed. The same set of circumstances occurred with each drive, both had a high amount of 'activity noise' even though the system was idle. After two-three weeks of hard drive issues, the S.M.A.R.T./PFA detected a potential failure and recommended to replace the drives.
You might wish to consider isolating each drive you own, to find out where the noise is coming from. If I were you, I'd then backup any essential data.