NO NO NO NO NO NO NO .
I USED to have that problem. It was that the seat was all the way back against the wall of the car. Scoot it up one or two clicks and your prob should go away.

...or it could be the seatbelt.

 

No, it's not the seatbelt
No, the seat is not all the way against the back

The problem is the seat back. The cushion is firm, however the seat back has some play to it, so does my driver side. When there is no weight against the seat back, it shakes in the little gap of play that it has. I'll mention it at the dealer next time I take it in, we'll see what they do about it.