iPhone 4 Vibrates For No Reason?

I’ve been noticing over the past few days that my iPhone 4 seems to vibrate very softly, sporadically throughout day.

At first I thought I was crazy. The vibration is barely noticeable and the first few times it happened I simply dismissed it thinking must be going crazy. Luckily I’m not.

Anyone else noticing a very subtle vibration coming from their iPhone 4? I can’t seem to recreate it on command.