I'm on a different device so I'm not sure how relevant this is, but there are a few settings for vibration on my device.
In 'Sound', in the sub heading 'Phone calls:' there are settings for 'Vibrate on ring' and 'Vibrate in Silent Mode', in the sub heading 'Messages:' there are settings for 'Vibrate with message sound' and 'Vibrate in Silent Mode', and in the sub heading 'Other sounds:' there is a setting for 'Other vibrations'.
Also in 'Language & Text' down the bottom there is a 'Keyboard vibration' setting.
I was getting the odd unwarranted buzz here and there until I turned off the 'Other vibrations' and 'Keyboard vibration' settings.