We are on a long road trip, and in the middle of it, we stopped at a stop sign, and I heard this cycling sound coming from under the hood. Took a video of it, hope I can upload it. (File too large, will upload after we get home). Have heard the sound one more time since then. Sounds like someone sawing wood with an old fashioned saw. Engine was running when I first heard it, then continued with the engine off. It seemed to last about 5 or 6 times, then repeats. Any ideas?
Could be the brake booster pump at the master cylinder. Usually it runs when you open the driver's door after sitting overnight but will run at stop signs and other times with brake actuation. Normally you don't hear it while driving because of road and engine noise.
If it is a Gen 2 Prius, and it is a going-back-and-forth kind of a sound, you might listen to the coolant control valve, near the thermos under the left fender. The engine computer wants to send the valve to a certain position; the valve's position sensor gets old and flaky and sends back wrong jumping-around signals for where the valve is, so the computer keeps trying to send it "back" to the right place. The brake pressure pump does not make a going-back-and-forth noise. It just pumps in one direction, and sounds like a pump. Or like one of those prank handshake buzzers.
Thanks for your reply. I have not heard the sound since, but have experienced other issues. I have made another post about them. If I do hear this sound again, I will pay more attention to where it's coming from. George
I have the cycling sound & it seems to be emitting from under the fuse block near left fender. Should the coolant valve be replaced? Will it prevent engine coolant flow?
Often, the cause of the problem is just the position sensor inside the valve. The rest is mechanically ok, but the computer gets flaky information about the valve position, so it keeps sending the valve back and forth trying to find it. You could follow this thread about the position sensor (a simple potentiometer) as an alternative to replacing the whole valve assembly. P1121 and inside the coolant valve | PriusChat
