What did you replace them with, OEM or aftermarket?
Slim possibility that a pully is going out, maybe, but I can't think of any rotating items that would cause this other than a clutch or flywheel issue, also maybe!
What is the engine idling at when you feel this vibration?
Tuxedo Black 2014 Focus ST3
CD Silver 02 SVTF, #1336 - R.I.P.
"Brace for the G's, and fast heel-toe work!"- Hunter S. Thompson