I have a 2009 5530 turbo. I’ve had it about 9 years. As long as I’ve owned it, the engine starts and runs well, sounding like a typical rattly diesel. It seems that when I work it hard and the engine reaches about 1/3 on the temp gauge, the engine sound instantly changes to a much quieter running. As the engine load decreases, it slowly goes back to normal sound. Any ideas. ?