Being all hard surfaces, the garage space was a big echo chamber - much like many a stairwell where I've experienced whistlers. I doesn't matter if the dealership is in the Midwest, Northwest, Southwest, or even East coast, I've had the same experiences every time I go to one to get my car serviced or look into getting a new one.. I'm convinced the more resonant a space is for sound to bounce around, the more it entices people to whistle, or make sounds to show off that echoing.
As far as certain professions being prone to creating triggers, over the years I've been involved in creative professions and activities and found that creative people like to keep active. Artists and other designers constantly doodle and play with their writing instruments (even if it's just flipping a pen around their thumb). Musicians frequently tap to the beat - even when there's no audible music - or whistle. Dancers frequently keep their feet moving, even while sitting in place. I could come up with MANY examples of situations where I've been around creative people fidgeting or creating other triggers.
What I find ironic is the number of books and articles that talk about how highly sensitive people are creative people as well. So if people with misophonia are highly sensitive people, and many highly sensitive people are creative people, and many creative people tend to fidget, whistle, or create other triggers... What is wrong with this picture?!?
Just more deep thought ramblings about my misophonia and life experiences.