Xbox 360 Pro 20 GB (Replacement from Microsoft after RRoD)
Within a minute of startup the rear exhaust fans on my 360 rev up to full speed, even at the dashboard. This makes streaming media and Netflix quite unpleasant. I don't really mind it during games, because the optical drive is loud anyways.
In case you were wondering, the Xbox is in a well ventilated area, standing up away from other objects. The console is sealed and has never been opened. It has never been dropped or mistreated.