I am at a lose with this one....

Okay this one as thrown me for a long time.
here it is, I am getting this odd vibration when at idle, its been there for maybe as long as I have had the car. Its kind of this vib.....vib.......vib......vib...... type of thing. It almost seams like it goee along with one full cycle of the motor. However I have popped the hood and put my hand on the motor while its running and don't feel anything . So any Ideas. One thought I had was it may be the flow of exsust moving throught the system?????