Here's what that single WD HDD sounds like (with background noise reduction) and what the waveform looks like.
I bought a Synology DS710+ NAS last week. It ships without HDDs, so I purchased two Seagate Constellation ES SATA 3Gb/s 2-TB Hard Drives (ST32000644NS) separately. This particular HDD model is on Synology's extensive drive compatibility web page.
The main reason I purchased it was I wanted a 2TB SATA drive that was quiet. Tom's Hardware - Benchmark Sound Level chart shows this Seagate Constellation ES model as one of the most quiet.
Well, unfortunately, although it is mostly quiet, it periodically makes what I consider a "thumping" sound. These drives passed the extended S.M.A.R.T testing and the very long NAS volume creation. However, this thumping sound really stands out, much more so than the usual sound of active hard disk drives. I've been around multiple PCs and file servers now for over 17 years, so I absolutely know what is typical for hard disk drive noise and these Seagate drives are making a racket I just don't expect to hear every few seconds.
Of course, only after purchasing these new Seagate HDDs, I searched and found a reference to the same sort of thing for an older Seagate 750MB (7200.10) HDD reviewed over at AnandTech in May 2006:
The drive is basically a "silent" component in our case during idle conditions with only the occasional disc whirling but at full load the drive does generate a noticeable, almost thumping sound. In fact, although two of the drives tested are considered louder by our test equipment, the noise tone was slightly heavier which elicited our immediate attention to it.
I used a Nokia N900 held close to the back of the NAS to record the sound. I did the same next to the front grill of my Dell PC which has two internal HDDs, a Samsung HD502IJ and a Western Digital WD4000AAKS.
Audio clip 1a: Two Seagate ST32000644NS HDDs in a Synology DS710+ at idle (no connection made).
Audio clip 1b: Audio clip 1a with 70% background noise reduction (Adobe Audition 3.0).
Audio clip 2a: Two HDDs in a Dell PC while doing a large file copy between the two drives.
Audio clip 2b: Audio clip 2a with 70% background noise reduction (Adobe Audition 3.0).
The sound clips with the background noise reduction are easier to listen to and hear just the hard disk drive noise. In clip 1b, notice the spike in sound at 2 seconds in and again at 37 seconds. When looking at the clip 1b waveform, from Adobe Audition, it is easy to see how much louder these "thumps" are than anything else. These spikes are just as easily seen in the clip 1a waveform, as well, which is the whole point that they are that much louder than the background noise (including the NAS enclosure's cooling fan).
Now, between 3.7 seconds and 4.7 seconds into clip 1b, the Seagate HDDs can be heard making noise again, but not nearly as loudly. The clip 1a waveform shows it is just a bit above the background noise, although it is still quite noticeable in the clip 1b waveform. The same sort of thing happens between 17 and 18.3 seconds and again between 24.4 and 25 seconds. At 42 seconds and until the end of the clips, the noise is louder, although not quite as loud as at 2 and 37 seconds, so sort of a "mini-thump".
Compare the Seagate HDD noise to the two HDDs in my Dell PC. True, the Dell PC case probably does dampen the sound a little bit more than the Synology NAS, but this being a BTX style chassis there is a grill in the front where I recorded clip 2a/2b and I can always hear the HDDs when sitting in my chair using the computer. Like I said earlier, after 17 years, I'm used to hearing hard disk drives working away, as they are mechanical devices after all. The clip 2b waveform does show where one can hear some HDD chatter between 2.8 and 4.1 seconds, as I was copying a large file between the two drives at the time of the recording (whereas I haven't even starting accessing the NAS volume yet to make them do any work). However, it is definitely nothing that jumps out at you and when one looks at the clip 2a waveform it is harder to quickly pick out the HDDs in the overall background noise.
To sum up, there is no perfectly quiet hard disk drive and in a small NAS enclosure I was even expecting to hear them a bit more than the two drives in my PC, but this periodic thumping sound of the Seagate Constellation drives is just not what I was expecting.
To make a long post even longer, I've had a Netgear ReadyNAS at work in my office since December 2009, on top of a table right next to where I sit. It has four, not just two, Seagate Barracuda ES.2 SATA 3.0-Gb/s 1-TB Hard Drives (ST31000340NS) and oddly enough the four of them together are not as loud as just these two Constellation ES drives here at home. I have heard some "mini-thumping" every now and again at work, but nothing that made me frown in disgust like I did after only a few minutes of powering up my Synology NAS at home for the first time yesterday. I suppose I'm also not as picky about annoying sounds at work (sort of the baseline) as I am at home.
