I have a 2002 Audi A6 Quattro 3.0L with 140k miles. Recently, it has been making intermittent noises after driving 30 minutes. It sounds like a engine rev without stepping on the gas. I also noticed that the coolant temp gauge has been going up over half way. On really hot summers, this may happen and I usually would turn on the heater and the temp gauge immediately dropped. This no longer works now. The temp coolant sensor was replaced at 122k. Any ideas on how to troubleshoot?