I recently brought my car in because of a mystery noise, which the technician couldn’t hear. They asked to keep the car overnight to let another tech check it out. Still no luck. They had me ride with the tech as he listened for the noise. As you guessed, it didn’t appear. Through all of that, these guys were patient, polite and assured me that when it shows up again, they will happily check it out. They refused to charge me. I appreciated all their time and effort to help me solve the mystery. (Of course, I am hearing the noise again…)