Ask your dealer if this is a recurring problem in MDXs or if it is just your car! If it's a known problem, ask them if they have a technical service bulletin on it and ask them to give you a copy. Then, before you take delivery of the fixed vehicle, ask to take the service person on a ride to see if the problem seems to have been fixed. Finally, if it seems to be fixed, make sure that they write it up on an invoice (although it should not cost you anything) and have them specify exactly what they did. I'd be concerned about this problem coming up again in the near future. Keep an ear on it, and take it back as soon as you think you hear it again. Don't be concerned about being overly picky, it's your car and you have the right to get one that's not defective. In fact, you might ask a local attorney who specializes in lemon law and similar problems to see what you need to do to protect yourself.
Hopefully this is all just alarmist reaction to a simple problem, but it's better to be safe than sorry. Also, remember that removing the dash could result in new noises. Don't be shy - keep taking it back until it's right.