Shooting tip: Go onto a bus and listen to the engine noise. IT'S LOUD. You will note that the engine is constantly changing gears/speeding up/slowing down/running at different engine RPMS. It rarely sounds the same for more than a few seconds. If you are shooting a master shot (one without any edits), the changing engine sounds will not be a problem. BUT, if you need to cut different angles together, I guarantee that you will be able to "hear" each edit as you cut from one shot to another. Unless you have a really good sound editor, stick with green screen with the bus not running...