In a lot of cases the only thing you can do is keep driving until it gets bad enough that they can figure out the problem. They're not going to rebuild the transmission for a second time for free on your say-so, and they are not going to pay another shop to rebuild the transmission for a second time on your say-so.
You don't realize how many kooks come into these repair shops with imagined problems, demanding free this or free that. If they gave in on every one they would be broke in no time. It's understandable when a shop won't fix your problem because they can't recreate the symptoms or find something obviously wrong.
I've just learned to drive it until the noise is so bad they can't deny it's there.
Putting 10-ply tires on half ton trucks since aught-four.