One problem related to engine rattling and whining sounds has been reported for the 2004 Subaru Outback. The most recently reported issues are listed below.
During early December 2010 my wife was driving the car and heard a huge rattling noise. She pulled over and we got the car towed to the dealership. We were told that the valve on the engine cylinder got damaged and as a result the valve stem has fallen into the cylinder. As the piston was moving, it got damaged. So we needed to replace two pistons. Which we did. This is a design problem as differential material expansion has caused the stem to fall and damage the piston. If this has happened on a busy highway could have been fatal. This problem has caused by bad design and using two different materials for engine valve and block. Subaru needs to recall the vehicles and fix this problem. The customers should not be paying. The car is very well maintained and had only 65,000 miles. Got it repaired. Subaru was of no help.