What kind of a noise is it? And where is the noise coming from? I remember in the past we had an MX5 that had a strange noise at certain road speeds/ Engine speeds and it was the rear exhaust box.

It looked like new but, the outer skin was vibrating.

What sort of noise is it? Is it blowing, any smoke? I would suggest you find another one to compare it with you will then have hard evidence to confront the dealer with.