TSB
There is a TSB (05-003-03)out on 03-04's built prior to December 5, 2003 for 'rear disc brake moan-like sound'. "The customer may experience a rear brake moan-like during light or no brake application pressure. The condition is more likely to occur whe the brakes are cool (first use) and ambient air temperatures are below 50oF. The moan-like sound may be intermittent."
Here's their solution: "If the condition can be duplicated or the customer describes the above condition perform the Rapair Procedure." The repair procedure includes NEW pads and rotors, a good cleaning of the calipers and new brake fluid.
I have this exact problem, but my build date is late in the 04 model year and I only have 16,000 miles. I've got a good mechanic and hope he will take care of it for me without too much ******** and complaining on my part.