vibration in my seat in my ford mustang?

[Gene]

I have a ford mustang base model v6 automatic and rear wheel drive.I have been noticing a vibration in my car that can be felt in my car seat ,and a little rumble with the vibration.it only does it when i drive in the morning then as the day goes on the vibration in the seat and the rumble go away almost completely go away.What would cause a vibration in my seat of my car and also the rumble that comes when the vibration is there.My car has 99,000 km on it and i dont think the timing belt has been changed on it ,any ideas as to what would cause a vibration in the seat accompanied by the little rumble