Since day one (2013 4.7 V8) I've been getting a buzzing noise that would come anywhere past ~3800-4000rpm. A/C on or off, and it'd stay all the way to redline. Told them about it with the first oil change, and now again at the second service, they kept the car for 2 days and tried to convince me it's the catalytic converter used in the new cars. I'm not convinced, any thoughts?