How to handle this situation with a loaner car?

ElVee

I don't know how loaners are handled nor what the service manager may say, but I personally would not even mention it. And if asked later, I'd say it just did that when I drove it and thought it might be normal since I did nothing weird.

It might just need oil. Badly.

Even some spirited redlining is not excessive. People don't do that enough when even test driving new cars, let alone rentals or loaners. So it's not like you suddenly went to a track with it and beat the hell out of it.