Took the Prius up to Mammoth Mountain this past weekend. Noted some interesting things about the car at high altitude which I'll post in due time. But something weird started happening, and that's the topic of this post. A weird chirping noise started coming from my right front wheel. It started happening one night when it was about 30 degrees out, so at first I thought it was temperature-related. Whenever I pressed the brake pedal a short "chirp" would come from somewhere by the right front wheel. The noise does sound hydraulic in nature - it's similar to the "singing" a spigot might make right before you close it. But it's very regular in pitch and duration no matter how quickly or slowly I depress the brake pedal, or whether the car is moving or not. At some point in the pedal's travel the chirp would happen. The brakes otherwise seem to be behaving as normal, and the chirp stayed with me all weekend. I drove home today, and the chirp is still there, although it's happening less frequently. When it first started happening I could be in park and repeatedly press the brake pedal and the car would go "chirp! chirp! chirp!" Now that I'm home, it chirps only occasionally. This probably means, among other things, it's not temperature or altitude-related. As it happens I'm due for my first oil change anyway, so I'll have the dealer take a look at it. Just curious whether anyone else here has heard this noise before.