Doublebase

If the transmission is covered under warranty I wouldn't hesitate to have it done, you just bought the thing and you shouldn't be hearing that noise. If it turns out it was just tires...well you now have a new transmission! What do one of these transmissions cost? 8k? More?

I really have no idea why a planetary gear set would go bad with 30k miles though, those should be good for the life of the car.

Devh

Not to come off as being overly cynical but I don't think dealerships solutions are to shotgun a fix by handing out new transmissions and hoping for the best.
They will generally diagnose the transmission by calling in a specialist before they authorize a repair of considerable cost.
I would be highly suspicious of any dealership that cannot diagnose and would replace perfectly good working parts thinking I'm getting new stuff all the while increasing the potential of goofing up the installation because of incompetence.