I need a kind soul or two to run a simple test for me to see if my 2007 4Runner is doing something weird. You need a quiet location and decent hearing, for me it was my garage.

Note, this is not the EVAP check noise!

Steps to reproduce:
1) Turn the key to On, but do not start the engine. Initially the 4Runner will make lots noises and eventually quiet down.
2) It is at this point that I hear what I think is a new noise, an electrical humming.
3) I crawled around and I'm pretty sure it's coming from under the vehicle, just above the transmission, pretty much straight down from the shifter. It's fairly loud with your head right there.
4) I can get the "tone" to change slightly if I shift through the gear positions (while the car is off) so I'm pretty sure it's related to the transmission.

I took it to the mechanic and he couldn't hear it (duh, his shop is loud).
1) Has my 4Runner done this the whole time, and I'm just now hearing it?
2) Is something broken or breaking? I have no clue what's sitting on top of the transmission, perhaps some controller or something electrical...
3) Did the mechanic screw something up when flushing the fluid recently?

brake booster?

I don't know about cars enough to tell if this could be it or not. I do know that applying the brakes with the car off (but in the On position) doesn't seem to affect the noise. Any chance you've tried to hear the noise on your 4Runner? Mine is an 07 4x4 SR5.

Hoping someone is willing to do this simple test for me!

Find a quiet garage, turn the key to the on position but don't start the car, wait 5-10 seconds, listen for humming by transmission.

I have an ‘07 Sport & happened to read this thread while waiting for my wife in a grocery store parking lot. Not as quiet as my garage but still pretty quiet, and I’ll try this again when I get home. At first, I thought, “Wow, I do hear it!” Then I realized it was the cooling fan in my head unit and that sound stopped when I turned it off. After that, silence. Will update once I try it at home.

Thanks Bluesky, I look forward to hearing about your garage results when you have time. (Definitely not the cooling fan on my end!)

Checked at home in my silent garage. No hum. Can’t imagine what’s causing it but hope you’re able to figure it out.

I have a 2006 4.0 @ 130 K
When I turn the key over but do not start, yes there is an electrical humm noise from the engine bay area.
No worries

This is a humming noise that continues as long as you have the key to the On position, yes? Not the initial startup noises that can last 10+ seconds and are quite a bit louder.

I just had my tranny fluid flushed so I'm concerned the noise is coming from the tranny area.