They are trained to react exactly as you describe. Be polite and courteous and ask that the car be brought out so that you can take the tech or service associate out for a test drive.
This way the issue can hopefully be duplicated.
All I had I do was have my service associate sit in my car as I started I and let the engine idle.
I got the same song and dance as you. "We couldn't find anything wrong." "We couldn't duplicate the issue".
We sat together in my car as i turned it over to start the engine and presto. Crickets.
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