Whine down on acceleration - getting louder

So I've seen a number of posts about a whining noise, and I'm pretty sure mine is covered under one of those posts, but I couldn't find it. Anyway....

2014 regular Prius (not C,V, or plug-in). Not sure of exact trimline. 110k miles.

While I'm aware that I should hear some sort of motor noises in general, I have one that has been getting louder over the last few months. Here are some notes about it:

- I don't hear it when driving on electric. The engine has to be running.
- It's most obvious when accelerating from a dead stop to get up to traffic speed. In that situation, it's most obvious when the engine gauge is in the PWR range. As I let off the gas pedal, the whine quickly winds down and becomes less obvious.
- It's also very noticeable when first turning on the car and the engine has started for warm up. Or if I'm stopped at a light and the engine is charging the traction batteries.
- It's most definitely NOT a grinding sound. As long as I've owned the vehicle I've heard various motor sounds (fairly quiet subway-like whines), and not worried. The only reason this one concerns me is that it's gotten considerably louder recently.

Based on my fairly limited knowledge, it appears to be somewhat correlated to MG1's RPM. I put a scantool on and watched it with Torque and semi-correlated the sound with MG1 RPM (not perfect, but Torque's display response may not be perfect). But I'm not sure what other components would also correlate (bearings, part of the gear assemblies, etc).

I've taken it to a transmission place, and they didn't want to replace anything unless they were more sure of what part it was. They wanted me to wait until it got worse. Pretty wise if you ask me, given that investigation probably would have required removing the transmission (tons o' labor), and they didn't want to replace one part, put it all back together and turn out not to be the correct part. They did replace the fluid, but it was only slightly off color and there were no metal bits coming out, so that's good. It needed fresh fluid anyway, so no worries there.

They did say that they could get a used transmission for about $800. Not sure if that is a good price, but looking for P410 transmissions online came up with a price of around $1000, so it may be.

The questions i have are:

- Given that the shop wanted to wait until the sound got worse, is that sound (no pun intended) advice, or should I persist more on the repair?
- Has anyone seen this particular issue before, and is the repair pretty easy?
- If it requires replacing a component of the transmission, would it be better just to replace the whole thing? Either way you'd probably have to remove it from the car, which (based on what I can tell) is the majority of the cost.


That's a lot of words. Any sage advice from the more knowledgeable here?