I noticed when I am on the freeway, or traveling around 70+ mph, my passenger seat vibrates when it is not occupied. My car does not vibrate or shake, and my alignment is fine. Is this a common thing?

 

mine does this too, and i'm sure that if you're crazy enough, you can see the driver's side seat shake without anyone in it at high speeds.

Not that i've ever put the car on cruise control at 70mph and hop into the passenger seat. no, no, would never do that...

though, maybe you could bring it up to the dealer next time you go? My dealer likes to deny that any problem ever exists on my car. i haven't seen a tsb on anything like that, with the exception of one that talks about rocking or sliding forwards and backwards of the driver side seat. that might be related.

good luck.

 

Get an alignment. Make sure the wheels are properly balanced.
If your lowered, camber issues can cause it.