The only thing I've seen frequently is the front seat belt being twisted
and the tongue whacking the pillar trim between the doors. Straightening
the belt out usually resolves this issue.
If the dealer can't hear it and you can make the noise occur, insist they
ride with you so they can hear the noise.
I once rode with a customer in a car making a noise I couldn't hear. We
got another technician to ride with her and he heard it and was able to