04 Freestar occasional slight vibration on highway
 

The past two months when driving above 45-55 mph I will occasionally (seems
to be every 3-4 minutes) get a very slight vibration somewhere under the car, and seems to be in the front. At first I thought it was the highway and I was going over a small rough patch, but it occurs on every road. It is very slight,
almost a mini shake feeling. I had the transmission torque recall done about 15,000 miles ago and that is the first thing that came to mind. Also, could it be
a bad tire rod, or ball joint going and even out of balance tire? It was just inspected one month ago though and passed. What is weird is
it shows up out of the blue as a very slight rapid vibration (kind of like going over the strip on the side of the highway to keep people awake but a much lesser scale), then gone again after maybe 2 seconds it seems. Can a transmission shimmy/vibrate? I read Ford had lots of issues with this tranny and reason for the recall, but I would think once repaired no issue. Not notice in town though, just on highway. I had tires going bad before but this is different as the vibration is inconsistent. Any info appreciated.