It's a long shot but ...
if there was a build up of light rust on the surface of the brake rotors except where the pads were in contact with the rotors while parked it may have been the grabbing of the pads on the rotors on and off the rusted surface as the wheels rotated and may have transferred a vibration through the car.
I had this similar situation occur on another car parked for several months but the vibration was only present when applying the brake and also eventually went away after a few miles of driving and cleaning away the surface rust