We bought a used 17 mdx advance about 6 months ago. Recently it started making a vibration noise at about 1800-2000 rpm right before tranny shifts. It sounds to me like a heat shield by the exhaust. I took it to the dealer yesterday, they had it all day and could not find the source of noise. Said they have never heard of this problem. Car has 36,000 miles. Anybody have a similar issue? Thanks Greg.