I have a 2010 Prius that makes a continuous high-pitched tone or alarm sound after it starts up. There are no indicator lights about a problem, and the car seems to drive fine. I took it to a local garage, where they ran diagnostics but did not find anything, and they're rather mystified. It may or may not be relevant that the 12V battery was recently changed, although it did not happen until at least several days later. Someone posted with a similar description on Reddit. The video they posted sounds like the what I hear: Any thoughts? Thanks!
I do not get any audio from the video you posted. Did you try to disconnect the 12v and reconnect to see if it helps?
From your description (not by listening), a continuous high pitched tone makes me think of the alarm for the system that keeps operating pressure for the brakes. Ordinarily it would also light a brake warning light on the dash. I wouldn't expect the tone and no light, unless there's a problem in the warning light circuit (unlikely, not impossible). Has anybody done a health-check scan with Techstream? Sometimes third-party scanners will miss something. It would be possible to have nothing more going on than a problem in the circuit feeding the high pitched alarm buzzer itself.
Thanks for the replies. @digfordirt, I have disconnected the 12V twice, once briefly. The problem went away for a little bit and then returned. On the second time, based on another suggestion, I disconnected the 12V and held the cables together for a couple minutes to try discharge any capacitance in the system, but that didn't work. The sound is loudest under the dashboard on the driver's side. Also, the video starts off with the audio muted by default, at least when I first encountered it. @ChapmanF I don't know what diagnostic gear the local garage has, but at this point it's going to a dealer to look at. Not looking forward to a 20-minute drive with that going on!
According to the dealer, it was the Skid Control ECU / ABS Actuator, which had to be replaced (and is not cheap!). But at least it's quiet now.
