I also have the 04 XC90. We have had issues with the front suspension since purchase 2 1/2 years ago. Initially it was a large clunk when turning which was a ball joint. After we replaced that, we could hear a slight rattle in the front end on the passenger side. We had the stabilizer links changed but the rattler persisted. My mechanic thinks it is the upper strut mount and bushing. You can actually grab the strut from under the hood and move it back and forth and it moves as the vehicle rocks. The sound has grown louder and louder. Good luck! If it gets louder, I would have them check the upper strut mount or the lower control arm bushing which needs replaced as well in my car.
Upper strut mount and bushing replacement less than $100.