Nomatter what he told you, go to the dealership and try another of the same model anyway.
If it doesn’t rattle, try a third. If neither rattles, show him. If the second “test vehicle” does rattle, it may not be abnormal even if not all like vehicles do it. Under those results, it may be a judgment call. That should not, however, prevent you from pursuing it as described below if you’re still uncomfortable with it.
And then pursue the issue up the chain of command as defined in the paperwork that came with your owners’ manual. Keep all your copies of the shop orders and write your complaint letters in a clear, articulate manner supported by copies of the shop orders (keep the originals on file). That way, if it should later develop into a problem, you’ll have all the evidence that you tried to address it while it was still under warranty and they declined to do so. Once something is reported under the warranty, they cannot refuse to repair it under warranty coverage even if the miles are beyond the coverage limit.