Solid, high-pitched tone on start-up

In the last month or two, half a dozen times now, I've heard a solid, high-pitched tone for maybe five seconds after turning on my car. It's a higher pitch than the beeping tone that I hear when the car is in reverse. It's definitely an electronic tone, not like something mechanical is squealing. No caution or warning lights accompany this tone. After about five seconds, it stops and everything seems normal. It doesn't happen every time, like I said, it has been maybe once every week or two lately.

Maybe I haven't searched hard enough, but I haven't found any certain explanation for this yet. Does anyone know what it is?

 

Do I need to try it in the morning? So that everything is cooled down? I will try it and report back.

 

@ChapmanF I was in a rush this morning, so I didn't get to test this. But I did have a couple of relevant thoughts: I've been driving the car for more than 15 years now, so I have an intimate feel for what's normal and what isn't. Over the last couple of months I think I've noticed that the pumps - the ones that run after shutting the car off, and (I think) upon opening the driver's door to get in and start the car again - seem to be running more. Also, once, and only once, I remember the brake pedal feeling funny when I had just started the car and was backing out of a parking space. Like maybe I had to press the brake pedal a little harder than usual, but then it seemed normal again. Both of these things would point to your theory, right?

 

I didn't realize it until now, but it's pretty old... I last changed it in 2017. Time flies.

Today I finally went to take out those ABS-1 and ABS-2 fuses and try starting up the car, but I got stuck on how to actually remove them. I found them but didn't see an obvious way to access the smaller one. To remove the bigger one, do I just pull real hard? I was worried about breaking something.

 

I realized that yesterday I was looking at the ABS No.1 relay ("the bigger one"), not the ABS-1 fuse. Today I found the correct 25A ABS-1 and the grabber and I was able to remove that. Now I don't see anything to remove where the diagram shows the 30A ABS-2 fuse, even after successfully removing the clear cover. I tried starting up the car anyway, and I got the ABS light but no audible tone.

 

Keeping with the pattern of about once per week, it happened again today. This time I did see the ABS light along with the tone. The light was probably on before too, but I didn't notice because it happens quickly. Tomorrow I have an appointment with a repair shop. I'm having them change out the 12V battery among other routine things, and I'll see what they think about the ABS.

I'll note that this has never happened after the car was sitting in the garage overnight. Like today, it has happened after getting back into the car to drive home after running a brief errand somewhere.

 

I got the car back from repair shop #1 today. They did some routine maintenance and diagnosed the ABS for me, but said that they couldn't actually do the repair on the ABS. Their notes are below. Now I'm trying to find repair shop #2...

Pending codes: C1253 Hydro Boost Pump Motor Relay Fault, C1256 Accumulator Low Pressure Fault. Voltage from the pump to the accumulator pressure sensor would repeatedly build up and then drop to 0 V. ABS actuator should be replaced and reprogrammed.

 

Here's the end of my story:

On the 20th I read a few other threads about the ABS issue and got nervous about continuing to drive my Prius. After calling half a dozen independent repair shops, nobody could see it in a timely manner, and two people said that I'd probably need a Toyota dealer for this job anyway. One dealer was able to take it that same day, so I went there.

They ran some tests that triggered the same C1253 and C1256 codes, and unsurprisingly said that I needed to get the whole ABS assembly replaced. It took them a week to get the parts and finish it. The total cost (parts, labor, taxes, etc.) was $2,800. That was more than I was hoping to pay, but they also gave me a "free" rental car for the whole time which was convenient and probably worth a few hundred dollars.

Driving my Prius home today, I noticed that the pump was running MUCH less than before. I think now that the ABS was struggling for longer than I realized, and I just got used to hearing it.

I know that a lot of people would have balked at paying so much to repair a car that's nearly 16 years old, and maybe not worth much more than $2,800 itself, but it was well worth it to me. I was just reading yesterday about someone encountering an 18-month waiting list to buy a new one... no thanks. I'll keep repairing this one for as long as I can.