[Donny]

All mass produced engines have spots in their rev cycle where they can be a bit harmonic and resonate some. The cranks were basically balanced to an acceptable factory standard, but not all the reciprocating parts are balanced as in a properly prepped and balanced motor.
Add age, klms done and how its lived its life and your likely to have a few harmonics here and there. The Ford straight 6's are pretty good, you should see how some older V6's are.. The Old V6 Capri engines were shocking things..
Some can be caused by exhaust systems too, or tune out of whack a bit, crap in the fuel or partly blocked or leaking injectors etc.. The possibilities of what it could be are nearly endless..

Unless its excessive, I wouldnt worry about it. The later B series on barra DOHC engines quite reqularly bumped and shimmied around idle..