I've been having a problem occasionally where when the engine is cold, there is a buzzing noise that sounds almost like an electric motor. The noise increases in frequency with the engine speed, so it's not the brake booster, the transmission, or anything else in the drivetrain. It almost sounds like I'm driving a remote control car, but it's not that loud.

As the engine warms up I can only hear the noise at higher RPMs, then eventually it goes away completely and the engine sounds normal. And sometimes the noise doesn't happen at all when the engine is cold.

I heard it this morning, and the temperature outside was probably 60 with a low around 40 overnight, so not unusually cold. AC is always off, but if I turn it on that doesn't change anything.

And ideas?

I would check the belt idler pulleys as the bearings dry out after awhile. You can loosen the belt and spin them by hand. Any noise or notchy feeling means you need a new one. You can also try spraying lube in the bearing of each one to eliminate this as the issue but it's only a temporary solution if one is bad. Same deal with the A/C clutch bearing except that's not so easy to lube.

I second the suggestion of checking the serpentine belt idler pulley bearing and tensioner pulley bearing, on mine it was the tensioner pulley bearing going bad and sounded much like you described and eventually it totally seized up, as suggested release the tension and check each bearing for not turning easily and smoothly and if one or both feels bad replace them, if you do try and spray the bearings with a lubricate be sure to not get it on the belt or the pulley surface that the belt rides on or the belt will possibly slip, but it can also shorten the life of the belt if the belt is still in good condition, if you do end up replacing either or both bearings go ahead and replace the belt as well unless it's been replaced within the last year or two.

If its a whining noise like a supercharger whine it could be from the alternator charging the battery the first couple minutes. My brother had this with his FJ until he replaced the alternator.

I just checked my order history, and it looks like I probably didn't replace the serpentine belt idler pulleys when I did the timing belt a couple years ago, so I'll look at those or maybe just replace while I'm in there. I used the AISIN TKT021 timing belt kit, so I replaced pretty much everything that spins with the timing belt.

The noise does sound a lot like my friend's supercharged FJ now that you mention it, so maybe it's the alternator (or maybe someone installed a supercharger while I wasn't looking. I would't complain!). Lately I've been using the 4runner as my weekend car and not driving it much, so it makes sense that it might need to charge the battery more than usual when I first start it. I keep an eye on the battery voltage. It's usually around 12.5V, but sometimes I'll hook up my charger if it gets much lower than that.

I just did my alternator this weekend. Had some similar noises and such. I could turn mine by hand and hear/feel some oddities in the rotation. Also my interior lights would flicker at night when I was at idle

If you're a little bit mechanically inclined, buy yourself a mechanic's stethoscope at your local auto part store for around $10 or so and you can usually easily pinpoint many odd noises, just be very slow moving and careful using it with the engine running, also even a good bearing will be much louder using this but should be smooth sounding when you find a bad bearing it will be extremely loud and should be obvious when compared to other bearings.

https://www.amazon.com/HARDK-Automot...-2-spons&psc=1

My bet would probably be pulleys or a belt. Have you checked your power steering pump too?