2012 Prius making humming noise when driving especially on highway

Check your oil level.

 

How many miles on it?

 

Bad wheel bearing is another avenue to investigate.

Does the sound change only with speed, or with engine demand? i.e. if you floor it, does that change the sound?

 

so, it does change with speed

 

Could be HV fan, fan rpm goes up w speed. Maybe there’s something lodged in the squirrel wheel case. Or fuel pump on its way out. I don’t have rear seats in mine so I hear the hv bat & fuel pump all the time as rpm goes up

 

I thought the HV fan increases speed with traction batt temp? I never realized it goes up with rpm

 

just temp.

 

That's generally a good sign. There's pretty much nothing back there to make a speed-dependent humming sound other than a wheel bearing.

If you have some way to recognize the pitch of the sound at known speeds (a tuner, or perfect pitch if you have it) you can do some more checking to be sure it's not something else. Rear wheel bearing was exactly what it turned out to be for me that time.

That thread ends with a link to a continuation where I had a front bearing go next. The pitch/speed relationship is different.

A ChassisEar turns out to be really useful in pinning down the culprit bearing.

 

My mistake, Mendel is right; fan speed correlates to bat temp. I was thinking of the PID torque app that you can customize voltage to go up as rpm goes up.

 

FWIW, I could never hear the hybrid battery fan, even when it was on speed 4 (as displayed on ScanGauge). I think there's speeds 5, and maybe 6?

 

I can hear mine sometimes on hot days when I'm stopped and the ICE is not running. But i can't say I've ever noticed it outside of that scenario.

 

I’ll say 6 according to hybrid assistant app, there’s speed 1 - 6 on it. It’s always set to 6 during summers in my car. Boy it sure is loud but my jams at vol 63 masks it.

 

Was this ever resolved? My Prius 2012 is making a loud humming noise from the rear. The faster I go, the louder it sounds. Feels like there is a drag to the car,and if I let go of the gas a high speeds the car continues to hum just as loud. Almost feels like I’m in a semi.

 

wheel bearing?

 

The great thing about humming noises from the rear is there really ain't much stuff back there to make humming noises besides wheel bearings.

Can you tell if, not just the loudness of the hum, but the pitch of the hum changes with the road speed?

Some very musical people might be able to hear the pitch and name what pitch it is. Some very nonmusical people might be thinking "pitch? what?". A lot of the great middle of people might not be able to name the exact pitch but could definitely say whether it goes higher with road speed or not.