Ok, maybe I should clarify a bit. What I'm feeling is more similar to a singular bump vs a 2 or 3 second vibration. It's like someone softly kicking your seat - just one bump at a time.

Sometimes when I'm stopped in in gear and have the heater or A/C on I will get a vibration but I think that has to due with lots of draw on the electrical system. Not sure if that is what you are getting SomeDudeAtHome... that doesn't bother me as much because it goes away as soon as I turn off the fan.