I've noticed my 2000 Audi A4 Avant (B5) with 2.8 30V Tip-Q's engine over the last couple years growing noise: a consistent muffled "rattling" that makes the engine sound like a Diesel. This sound is most prevalent at idle. The dealership's top mechanic listened to it with stethoscope and says it is not valves or water pump. Rather, the noise comes from the center of the engine just below the exposed intake runners/ manifold. He says the only thing there is a plenum of sorts containing some valves? We are both stumped on noise. I do frequent oil changes of Mobil 1 Synthetic since I bought car off lease at 50K miles. Engine and bay are as clean as showroom. Thanks in advance of your help.