I purchased my 5th gen SR5 with 39K on it back in December 2017. Last spring (May 2019) at 58K, I started the truck up in the garage and heard the whine at 1000 RPM and up out of no where. The day (weeks, months and years) before the noise was not present. I have an extended warranty on the truck so I asked the dealer to take a look at at my next oil change (only about 600 miles later). They tested the alternator output (normal), compared it to another same model on their lot and chalked it up to normal noise. Based on all that I've read on this thread and another similar thread, I decided to let it be and not worry.
Fast forward to my next oil change (just a month ago at 67K). For kicks, I went to a different local Toyota dealer to see what they had to say. They bit on the extended warranty work right away. They verified that they heard the whine and seemingly willy-nilly replaced the alternator as the "fix". Again, based on what I've read, I had a very strong feeling that this was not going to be the fix but was happy to take a new alternator anyway. They gave me the truck back claiming noise was gone, but, of course it was still there.
Fast forward to today (March 6 2020 at 67800 miles) when I was able to get the truck back in and verify the noise with the tech and service advisor. They heard the noise with me. I explained that via this forum, it seems they should remove the belts to isolate the issue. They did this and the noise was still present. They've now decided to order an oil pump for the truck under warranty. I expect to bring it back in for the swap late next week. There's been a few guys on this thread that have had the oil pump replaced and that seems to have fixed the issue. I'll post after the work is done and confirm if it fixed mine or not.
As always, thanks to y'all for taking the time to share.