Current Fluid Bearing drives whilst generally being very quiet from a high-frequency vibration standpoint seem significantly more prone to low-frequency vibration compared with older ball-bearing based drives.
I've no real evidence for this other than my own experience, which is that every single new FDB drive I have come into contact with over the last couple of years exhibits a distinct lower frequency hum when fitted into a case, and is even noticeable just holding the drive in your hand.
The drives include Maxtor DM10's, new Samsungs, Maxtor Maxlines, Hitachi 7K250's. All of them to a greater or lesser extent seem to suffer in this way compared with the older generation of drives.
For example, I have several old 4.3G IBM deskstars which emit a pretty nasty whine, but hold them in your hand, fit them into a case ... there's virtually no low frequency/large amplitude vibration comparatively speaking.
The same holds true for several ball-bearing based WD400BB's and a 40G IBM deskstar 60GXP.
A friend who bought a pair of FDB Samsungs last year for a Home Theatre PC had no end of grief trying to stop his Thermaltake case from vibratiing with them fitted. Recently he wound up buying an Antec case supplied with rubber suspension bands which has sorted his issue out.
Perhaps FDB technology will improve further and this aspect of their behaviour will lessen, but at the moment, from personal experience at least, its not the panacea it might initially have seemed.