High-pitched electrical humming sound from the hood in Ready mode

My 2010 Prius has recently started having the same issue! It sounds like it’s coming from around the water pump area. If anyone has any definitely ideas of what it could be please comment!

 

i have same buzz as video below.
just changed inv coolnt pump and rear brakes ( sadly no techstream did not unhook 12v ).
coukd it be pump ? sound coming from back by actuator...

 

this sound comes on both when ig on , and in ready...

 

A mechanic's stethoscope is simple, super cheap, and easy to find at the local auto stuff place.

If you have never used one, you will be amazed at how easy it makes finding such noises. Put the ends in your ears, touch the tip to things (careful not to get it caught in belts or spinning blades) and poof! you know.

I suggest touching the tip to the small electric motor on the side of the throttle body. Strangely, a lot of people without stethoscopes still want to believe the sound is from something else. Perhaps yours even is.

After that, you can also touch it to various other things that aren't the throttle body, if you're still unsure.

 

6.20121 - Thanks Chapman, We listened and it was throttle. To confirm, We unclipped it and noise went away. Recently had throttle cleaning done. I thought they would remove housing to clean but they didnt. Might there be any issues to correct or tests? thx cv

 

Dang that is one clean car. Listened to mine with stethascope and it was throttle body. unclipped it and noise went away. Recently had cleaning. Also changed Inc coolant pump, so we were wondering. Never heard noise before.

 

It's not a problem. The throttle is positioned by an electric motor. The motor can be driven "this way" or "that way" by changing the polarity of the voltage across it. The engine control module gets the throttle to "hold" at the current intended position by sending it rapid pulses of thiswaythatwaythiswaythatwaythiswaythatway (if it just stopped sending pulses, the spring in the throttle body would take it back to a failsafe not-quite-closed that allows the car to be slowly driven).

The electric pulses are sent at a frequency that falls in the audible-to-humans range. They naturally make the throttle wiggle very slightly thiswaythatwaythiswaythatwaythiswaythatway at that same frequency. Mechanical wiggles at a human-audible frequency are ... sounds!

There could be various reasons for not having noticed the sound before, but the ECM and throttle body are always doing that, whenever the power is ON.

I think it's kind of funny that the ECM bothers to keep sending pulses to hold the throttle in a given position all the time the power is on, even when the engine is stopped, but that's what it does.

 

I had not noticed this sound before on my 2012 Gen 3, until today. I took apart the intake and EGR Pipe, cleaned both, cleaned the throttle body, etc. I may have been a bit aggressive with the throttle body cleaner... I initially thought it was a vacuum leak, or the water pump struggling... but was glad to find this thread, and the explanation. Thanks, ChapmanF. I'm going to keep my ear out for this and see if it changes, hopefully just one of those "beauty marks" of a 160k / 11 Year old Car...