I hear a rattling sound coming from the passengers side of my car when I drive

Make sure the nuts that secure the seat rails to the floor are tight.

I tightened the nuts to the rail. It did not work. Is there a way to tighten the seat back? Thank you

Front seat or rear? The rear seat seat back is bolted at the bottom. You'll need to remove the bottom.

The front seat back pivot points may be able to be tightened. I don't have one handy to look.

It is the front seat

See image

Sadly enough it may be your ball bearing on the front right wheel if you feel the vibration on the floor or the gas/brake pedal.

I'm guessing the sound goes away when somebody sits in the passenger seat.

Yes the sound does go away when someone sits in th seat!

The Rattle on my 2007 Hummer H3, that sounded like it was behind the glove box, was vibrating at lower engine speed. Turns out my heater hose was laying against the transmission dipstick causing the heater core to vibrate inside the car. So I just bent the dipstick forward about a half inch and it fixed the noise. I was a piece of cake.