IPhone randomly starts vibrating?

Sometimes when I switch to vibrate it'll just randomly switch to sound again even when I don't touch it. And sometimes even if I'm not touching it at all, for minutes it will repeatedly just vibrate and keep switching from the vibrate setting to the sound setting, and it's quite annoying. It's a 3Gs, so it's fairly old, but is there anything I can do to stop this from happening?

Could be notifications alert messages e mail etc.go to settings and go to volume there should be a few volume options