Being adamant is the key here. If a dealership says it's normal, just say "OK, find me a used 201x GT on your lot, and I want you to take test drive with it right now to prove that it's normal".... Go for the test drive and listen. If same noise isn't there, it's NOT normal. Try 2 more used GTs just to make sure.
Also ask them if they'd sell a used car making that same noise, or offer top-dollar on a trade in if it had that noise..... because it's "normal". Try doing a trade and say that when they question the noise at trade-in time. See what they say THEN.
That's the key point right there! It's a one way street with these dealerships.