My G35 sedan recently started making a strange vibration noise at speeds of 50 mph or higher. It sounds like a deep metal friction noise, but it could also be a wind vibration noise. It sounds and feels like the car suddenly starts moving on a rougher surface, and the gas pedal also starts to vibrate as if that was the case, but it happens on any surface. It seems to depend only on the speed of the car - the problem goes away when the car slows down. Steering the car or braking has no effect on this problem. I took it for a ride with the mechanic at the local Infinity dealer, who acknowledged the problem, but he has no idea what causes the problem. Has anyone experienced this problem? Any ideas or suggestions?