Question: My 2004 Infiniti QX56 has more than 90,000 miles. Last month, I started getting a metal-on-metal noise from the front when accelerating from a stop. I also hear it at about 45 mph, if I hold my foot on the gas at just the right spot. It's a more steady noise this way.
Answer: I can't be sure without hearing the noise for myself, but your description leads me toward a heat shield rattling on the exhaust. It's kind of a tinny rattle, almost a buzz. Heat shields are placed strategically across the exhaust system to protect components and the underside of the body from heat damage. A common problem in northern states is that these shields corrode, become loose, and rattle like crazy because of road salt. Heat shields are made of a very thin, lightweight metal; therefore, they're highly susceptible to corrosion. And, trust me, it's not just a QX56 issue. It happens on most brands and models, after time and mileage in the road salt areas. Usually a quick visual inspection of the exhaust will spot the problem area. Often the shield will rot out right where the bolt or bracket secures the shield to the exhaust. Sometimes it does require replacement of the entire shield. But you may be able to save some money by re-securing the shield with stainless steel clamps. Noises like this often coincide with normal engine vibration at specific engine speeds. A good diagnostic test is power braking (accelerating in gear, brakes applied, wheels chocked). This allows us to take the engine through an rpm range while under a load, and duplicate that same vibration level that triggers the noise on the road. Once the noise has been duplicated in the shop, it's usually an easy job pinning down the cause from underneath the vehicle.
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