Knock on driver side help

I Have a 406 1997 and it has developed a knock on the drivers side at the front, it sounds like a tin of beans under the seat rolling and hitting the framework of the seat, but I have had a look, no tins. It happens at iregular times, perhaps on acceleration but not always if you turn sharply it may do it, sometimes it just may do it when changing direction. It only does one knock at a time so it should rule out the drive coupling, if anyone has had the same problem or you need any more info just ask....kind regards Mick.