Vibration analysis. I used to do it at GM for preventitive maintainance. I had a hand-held reader with a wand that read the vibration. Download to a computer and it told the frequency among other qualities of the vibration. Once you have the frequency you could tell what was vibrating. The frequency matchs the revs of what is vibrating. You can do this to an extent with your senses. Try to distinguish the cycle of the vibration and it will give you a clue.