Ok, final quiz: :)

a) is it a short, very short vibrate, or is it a longer one? By short, I mean the kind of vibration you get when you tap the screen.

b) Does it happen if you disable vibration? It should be disabled in profile settings (both profiles) and the vibration on tap (haptic feedback)

c) Did it happen just after reflash or did you install ANYTHING? It could be worth reflashing completely (root + home) and just wait an hour to see.

d) If all the above are misses, you're down to system logger and kernel logger. Wiki entries describe how to install them, and make a dump. With that dump you can be helped (not by me, though), people around here can tell you if it's a software problem or a hardware issue via the log. One that has been established, you can be directed to a Nokia Care Center or to Bugzilla, depending on the nature of you problem. Neither accept ambiguity, so this is a necessary step.

Try A through C first, it's a lot less hassle.

Good luck.