noise when car is going down the road sounds like running over a rumble strip on 2003 Mercury Grand Marquis

the noise is very light you feel it more than anything and it is when you let off the gas sometimes or when you go back to accelerating feels like you are running over a rumble strip in road but just very lightly what could be the problem

if you notice it when vehicle shifts into overdrive, around 45 mph, transmission needs to be flushed.