Random Vibrations.

[Leeroy Jenkins]

Could it be that your hitting the volume button and it's causing it to go from Ringer VOL.1 to Vibrate and then back to Ringer VOL.1?

I've had this happen to me before.

[The Coon.]

Do you know it's the phone vibrating? Or could it be your leg doing something weird which makes you think it's vibrating. Happens to me mostly too.

[DeFacto]

Mate, why the fudge did you take my vibrator again??

[Nexum]

It could be that ghost vibration thing?
Or is it vibrating and you can clearly see it?

[revest]

As most of the above posts say,
I would suggest to take out the Battery and then put it back in.
Or take it of vibrate (:

Hope this helped, have a good day.

[Party]

Call your phone company, and/or punch it a few times, and set vibrate off.

[AMD]

My phone does the same. Its running on android 2.2. The phone must be having issues with a app or its crashing if its a android.
Samsung Intercept VM

[Pondake]

It'd help if you tell us what phone you have.

[Radix]

My Iphone does this. I just ignore it, then eventually will restart my phone. Doesn't happen too often and doesnt affect anything so I never cared.