Just noticed I forgot my manners. Welcome, you will find a great bunch of people here who always try to help.
You might want to search on Dolphin Noise or National noise. I did a quick search and came up with this thread http://www.irv2.com/forums/f105/whistling-noise-dolphin-86362.html
It looks like there was a know problem with the grill in some of the Dolphins. Not all of them whistled some hummed If you are even slightly mechanical pull the grill and take it for a drive. Noises are a pain to find some times.
Can't help you with service in that area the only work I had done was at Freightliner of AZ in Chandler who do great work but probably not what you need.