Hi,This will most likely be an issue with your belt drive on the engine that needs addressing most likely. It is an issue with load amount on the engine at lower engine speeds. On earlier versions of this engine you could get an updated modified tensioner/idler for the belt to increase tension on it; being a 2012 you may already have the updated part but it wouldn't be a bad idea to check with your dealer to see if it is already there or not based on the build date.
What I would want to do at this point would be to take a crayon or sliver of bar soap and run it underneath the groove side of the belt while the engine is running to get some of the wax/soap embedded in there, then take a drive. If the noise changes at all it confirms the belt area is at fault like we suspect. At that point if the belt is original you can try replacing the belt (use a genuine Toyota belt only!) and compare behavior, however if the noise persists then you will likely be looking into a tensioner/idler replacement, whether you have the updated ones already or not. Given the history of belt squeals on this engine, odds are you will be ending up with a tensioner and idler replacement though.