Sorry to make this sound stupid but,
Your at the dealership right? The guy says it is normal, humm lets drive one of his trucks and see if the sound is there? If his truck makes the same sound well, it probably is normal.
If not offer to trade your normal truck for his abnormal one. Don't keep taking crap from these guys. Ask to speak to the general manager and if that doesn't get it then get the Regional Service manager involved.
Don't forget to keep us up to date with the resolution
