In response to this shaking, this same thing happened after I got in the car and turned on the engine. My son and I were scared. It felt like the car was being pelted by large hail stones (on a clear blue sunny day - why did I think this was happening?) The speakers were booming and the car was shaking. I turned off the engine and opened the door.
After our hearts calmed down, I restarted the car.
Recently I had brought in the Focus because the radio controls would go all the way up (high volume) or down. No in-between. The reset the whole system.
I'm not too happy with all the problems - and this car is less than a year old.