Why my D2 vibrate when it should not

[BikerTech]

Just upgrade from D1 to D2 over the weekend.

gMail settings:
Select ringtone: Silent
Vibrate: Never
Settings > Sound:
General > Vibrate: Never
Notifications > Notification ringtone: Bells

Whenever an email (gMail) arrives, my D2's notification goes off (which is expected from the setting above) and vibrates (which should not happen). Could anyone explain why the phone vibrates? Thanks in advance.