My 18’ TRD does the same thing. I just noticed it and wasn’t sure if it was normal or not. This video is exactly what mine sounds like.
YouTube
All the replies say oil pump, power steering pump, alternator, transmission, rear diff etc..🤷*♂️
I don’t want to get scammed at a shop so I’m going to do a fluid flush and if it doesn’t get louder just pretend it’s normal and turn the radio up. If anyone knows the true answer please let me know.