My Prius Is Really a "Hummer"

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My ’04 Prius makes a sound that I’ve been unable to troubleshoot.

Description of sound: Sort of high pitched and something like a hum. It somewhat reminds me of sounds I’ve heard from air or leaking out of a tire or balloon, capacitors, solenoids, leaky vacuum seals, and high-tension power lines. It’s not a scraping, pulsing or relay type sound.

Conditions when it occurs: It’s most noticeable at low speeds. I believe its audible at high speeds as well, but I’m not yet 100% certain due to road noise. It occurs whether accelerating, coasting or braking, but gets louder when I step on the brake. There’s no sound when the car is at a standstill.

Frequency: Intermittent, but often enough to be bothersome. It can come and go during a single trip.

Location: I first thought it emitted from the dash area. However, listening with the driver’s window down, I now believe it may originate from outside the cockpit (engine, brakes, ???). I can’t say whether front or rear passengers can hear it, as I drive my Prius alone.

The following have no effect on the noise:
--Opening or closing the windows, so it’s not the window rubber weatherstripping;
--Having the A/C and/or fan on or off

I read a message from someone whose coolant reservoir cap made a sound. Could that be what I’m hearing?

I find the sound to be very unpleasant to the ear and would like to get rid of it.

Has anyone else had this problem? Any ideas concerning what’s causing and how to eliminate it would be very much appreciated. Thanks.

(USA vehicle, Package 9, Millennium Silver, 1,700 miles)

 

Thanks to all who’ve responded to my query.

I am familiar with brake noises and have had some doubt that the sound I am hearing is coming from the brakes. My hunch is that it actually is air. Nevertheless, the sound I am hearing my be related to what was reported in the recent “Squeaky friction brakes - is this normal?†messages.

Does anyone else have any ideas about what is causing the “soft whiny high-frequency hum†when I drive?

My plan is to have the Toyota dealer look try to solve the problem. However, I want to provide as much information as possible, especially since the sound is intermittent.

Regards