ive got this really anoying noise under the pasenger seat...the only way i can describe it is like this..... imagine you have aerosol paint tin with a ball bearing in it strapped to the underside of the seat and every corner you drive round the ball bearing rolls from one side of the aerosol can to the other......?????? what could this be????? its doing my head in

 

maybe you have aerosol paint tin with a ball bearing in it strapped to the underside of the seat ............
seriously tho sounds like a marble or something in the floor strut which the seat bolts to.

 

Strangely I had the exact same thing in my mrs' impreza! It was doing my head in, kept mentioning it to her after I'd driven it but she'd have the radio on and not notice.

In the end it turned out to be a one of my sons toys, it was infact a ballbearing inside one of those clear plastic games (where you have to locate the ball in a cup/hole) had made it's way under the mat right at the front of the passenger seat underneath. I'd looked /felt under the seat a few times but it'd hidden from me for a while

 

One of my mates thinks it could be the ball bearing that has come loose in the seatbelt pre-tensioner.. does this lay sideways across under the seat or floor mat or something?

 

Stone in the plastic under tray that runs under the pasenger side i expect.