I've done a search on this and have not found a similar problem, so: My 2008 Touring Prius has just under 9,000 miles on it. About 2 weeks ago it started to make a high-pitched electrical humming sound that is quite noticeable. The sound is constant no matter whether the car is in electric-only mode, accelerating with gas/electric, or at a stop with or without the ICE running. The only wrinkle I have found is that when I turn off the A/C the pitch of the hum lowers. This leads me to believe that the hum is related to the high voltage supply. This is my second 2nd generation Prius, so I am very familiar with the usual humming sounds, coolant transfer pump etc. I will have the service people look at it at the next service, but wanted to see if anybody has noticed a similar issue.
On one occasion, at a stop light, I noticed a high pitched hum and vibration. I thought that the driver to the right of me, also stopped, had a large boom box going in his car, that was causing the vibration. He looked over at me as if to ask, what's going on? I then realized the hum/vibration was coming from my car, not his. After I started up from the light, the hum and vibration disappeared and hasn't reoccurred. I would inquire to the dealer if it did reoccur, which it hasn't. Have no idea what caused it. I have a 2007 Prius hybrid. Purchased used at about 19,000 miles in Nov. 2007.