Screeching Owl Noise Under The Hood

I have a 2006 Prius that has 102k miles that is beginning to make a lot of screeching-grinding noise under the hood. This is true between the speeds of 45 and 60 miles per hour. If I go over 65 miles per hour, the noise disappears. If I go under 40 miles per hour, the noise is not present. If I accelerate or go uphill, the noise diminishes. If I decelerate or go downhill, the noise increases. It makes no difference if I sway back and forth on the road, so it is not a wheel bearing issue. I believe it is the bearings in the transaxle and the fluid needs to be changed. The fluid has not been changed in 60k miles since I purchased it. When I take the car to the Oxnard Toyota Dealership in California, they (two Prius Techs) say that the fluid in the transaxle is sealed for the life of the car and never needs to be changed. They say that the transaxle is made of a bunch of magnets and there is very little fluid in there so lack of fluid is not an issue or the problem. They observed no cracks in the transaxle housing and believe the fluid level to be fine. They refuse to check or replace the fluid. They told me that they want the problem to get worse so they can diagnose it. They hear the noise as a "harmonic" noise and think it is electrical in nature. They erased all the old codes on the car and want me to bring the car back to them so they can view the new codes.

I think these Dealer Prius Techs are incompetent ! Is the noise coming from the transaxle and the lack of clean fluid ? Does anyone know of a good non dealer garage in Northern California to take my Prius to so I will not have to sell my house to pay for it ?

THANK YOU FOR YOUR RESPONSE !

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I had Luscious Garage in San Francisco change the Transaxle Fluid. THANK YOU FOR THE SUGGESTION ! John there said that there was no excessive metal shavings, etc, on the magnetic plug. It did not completely solve the noise problem, but the car is making much less noise than before the fluid change. There is a noticable change. Can I use Lucas Transmission Fix or some other transmission additive in the transaxle as an attempt to reduce the noise further?