Hitachi 2T HD vibrates in idle

[13mh13]

I have accumulated about a dozen Hitachi 7200RPM 3.5" HDs over the past 4yrs, ranging from 640G to 2T. They have proven to be mostly reliable.
I just purchased their latest 7K3000 2TB (reviewed by SPCR here: http://www.silentpcreview.com/article1243-page1.html ) and installed in the my usual USB AMS Venus external case (for testing/evaluating, one can carefully operate with this case's cover removed.)
To evaluate physical performance as well as troubleshoot, I always do a "hand test" -- place my hand lightly (gently) on the drive and feel for heat and vibration whilst it is idling/reading/writing.
Something weird I've noticed with this new drive is an "idling vibration" that feels like drives that are INTENSIVELY reading/writing. I can pull the USB cable (disconnect) to make sure the drive is NOT really reading/writing, and the vibration remains. Plugging USB back in to the PC, and actually doing some reading/writing, the drive becomes absolutely vibration-free in-between read/write cycles.
You can think of this issue with the following analogy: it's like a nervous leg-shake habit for an over-caffeinated office worker waiting around ("activityless") for their next assignment. One he/she is busy again, the nervous leg shake ceases.

EDIT:
I moved the "vibratey" 7K3000 2TB to another external case (made by AMS Venus, but a newer model) hoping the 2nd case's sata mother/driver-board and/or firmware may be culprit, but the vibr. issue remains.
I'm not sure exactly what "vibration" L. Lee measured for this drive when he tested it in Feb 2012 (it rec'd a poor "5" for Vibration).
Again, the vibration is slight but it is not a mechanical issue (out-of-balance, etc) because the vibration is not constant: it disappears when the not idling but also not seeking/reading/writing ... or when you plug in USB and the PC BIOS is "plugging" it in (vibr disappears) ... it's that hard-to-pin-down (or put into words) "in-between" state.
One more example: playing music or video file, this HDD cycles between vibration and non-vibration -- methinks this back/forth is due to normal caching. If I pause the music or video player, the constant vibration returns. The music files toggles vibration/nonvibr. much more frequently than the much-larger AVI video file. It's almost as if this HDD's default state is "constant" read.

[13mh13]

Again, the vibration is slight but it is not a mechanical issue (out-of-balance, etc) because the vibration is not constant: it disappears when the not idling but also not seeking/reading/writing ... or when you plug in USB and the PC BIOS is "plugging" it in (vibr disappears) ... it's that hard-to-pin-down (or put into words) "in-between" state.

Wanted to clarify this remark ... by "not a mech. issue" I meant not a (physical) mech.
defect. It's actuator read/write arm activity and the related vibration that generates. The magnitude of the vibr. is similar to to other Hitachi 1T or 2T I own, maybe even slightly less (makes sense as this is a relatively new model). The outlier issue is that the drive "defaults" to "nervous" arm activity in its "idle state" (somewhat analogous to a uni-cyclist having to wobble back-and-forth to stay "idle".
Wonder if poor formatting may be an issue? When I orig. installed, I performed a slow fmt after initializing the drive (my usual procedure). Any clues to whether tweaking with Hitachi's feature tool would help?