Engine replaced, but new issue?

Hello all - I hope you can help with a dilemma I'm having with my 2014 Prius Plug-In.

I bought the car used three years ago with 133K miles on it. It ran fine until I started experiencing engine issues.

I replaced the engine in February and right after I replaced it I started noticing I slight "whirring" noise, especially when I coasted in drive and "b" mode. I especially hear it when I'm driving over 50 mph.

I've been told by at least two sources it is the transmission, and one transmission shop told me it could run like this forever, or could go out at any time.

I'm sure someone besides me have had this issue, and am hoping I can get some idea what my next step would be. FYI, the car currently has 260K.

Thanks for your time!

Mike in Tacoma