Prototype

When ever I do a normal acceleration, to get the Five Hundred up to speed, there is a minor rumble noise/vibration coming from below the car (like driving over small rumble strips on the highway). The speed that it occurs at is always the same (55km/h to 60km/h), and it only last for about 2 to 3 seconds. If I accelerate harder through the 55 -60km/h barrier, no noise, if I let go of the accelerator prior to the 55 - 60km/h range, and then accelerate through the range, no noise. Has anyone else had this issue? Any ideas what it might be? Any feedback would be very much appreciated.

Thanks,