Transmissions cannot tolerate a slipping condition for very long, without total failure.
Slipping clutches will burn up within a few minutes to an hour of use.... and the slip gets progressively worse... so yo umay not have a slipping trans.
In your situation, possibly the torque converter is bad.... it no longer locks up and/or has a seized stator, etc. Such converter failures sometimes produce a distinctive noise and reduced mileage. And as far as I know, no code would be produced. Or possibly you have lost the overdrive gear.
Also, beware that sometimes such a converter failure can pump metal shavings/particles throughout the transmission. If this has happened, the trans will fail prematurely, and needs to be replaced.
I would say that it is NOT normal for a trans to start producing new noises. A transmission with lots of miles should produce the same noises as a new one. IMO any new noises indicates a failure of something.
This truck is still under warranty, right? IMO you should start making some very strong complaints to the dealer... possibly get a second opinion. But definitely ge t it remedied before the warranty runs out.