When the APM value of the drive is set to 254 or 255 (head parking is disabled) using either hdparm or HDDScan utilities, my HGST 7K1000 HTS721010A9E630 hard drive is occasionally making normal quiet activity sounds, but in periodic short bursts lasting (hard to say…) from 0.1 to 0.5 seconds and occuring every 1-3 seconds. The problem is thoroughly described in another question here. If head parking is enabled (APM value is 128), the drive clicks after it's been accessed, and no further sounds are heard until the next access to the drive.
I don't know whether changing the APM value using Hitachi Feature Tool will give the same problem, but I'm not willing to use it yet, until I figure out whether using it might void the warranty. I asked a question about the warranty both here at Super User and at WD Community website. No answers yet.
Best Answer
After several hours of experimentation, thinking and reading I came up with an answer. I have a rather crazy, but plausible assumption that the makers of HGST 7K1000 HDD were a bit lazy, and instead of implementing the disabling of head parking, they made the drive to make some activity (read something, I guess) in order to keep it active, so that the timer of inactivity never reaches a certain point, thus the drive is making activity sounds about each second. So, in other words, my guess is that it was easier for them to make a timer to create activity each second, rather than to change the timer which parks the heads.
Also there is a less crazy explanation, but still a bit strange: setting APM value to 254 or disabling the APM puts the drive into Active Idle mode, and it is supposedly normal (or designed this way) that drive is active each second when in Active Idle mode. Well, so that the drive doesn't become lazy and fall asleep on its job, I guess.
Here are some APM values that I have tested with the drive:
"The advanced power management levels contain discrete bands, described in the section of Set Feature command in detail."
"When Feature register is 85h (=Disable Advanced Power Management), the deepest Power Saving mode becomes Active Idle."
I provided the quotations from the HGST Travelstar 7K1000 specification in quotes.