Poping noise from inside dash

What is your question?

While under heavy acceleration at about 30-40MPH there is a distinct poping sound eminating from the dash. It sounds like a vacuum valve closing. It has been back to the dealer 3X's and they have been uable to duplicate the proplem. As soon as I pick it up and drive it away it will continue making the sound. No operation problems as a result more of an annoyance. Any suggestions what might be causing the noise?