Crescendo high pitched noise from engine

TW,
I have a double cab long bed 2006 Tacoma with 197,000 miles on it. Had it since new, and has never cause me any problems. Love this truck.
Recently, I have noticed an intermittent high pitched crescendo sound coming from my engine while it is running. Occurs at idle and while going down the road. No other evidence of problems. Drives fine, and the dash gauges indicate no issues. I have regularly done the maintenance throughout its life on time. Any ideas?
Thank you.

 

Maybe the belt or pulleys?
My OEM belt went about 12 years before it started making a squealing sound on cold mornings.
Changing the belt fixed the problem.

Just taking a guess here.

BTW, I have no idea what a crescendo sound is?

 

I was lmao at the crescendo portion of the title...still laughing

 

And while still laughing, wondering if a high pitched wail like right before it STB...but honestly was thinking belt, pulley or idlers

 

Just attempting to describe the noise accurately. Enjoy yourselves.

Belt is fairly new. Pulleys checked out when I looked at them 7k miles ago.

 

I had a similar issue just a week ago. Had this horrible high pitched whine but no indication about what was wrong. About a week later, all my dash lights reset while I was going down the highway with my battery light staying persistent. Ended up being a bad alternator. Replaced it and the whine was gone immediately.

 

I replaced the alternator in May 2017, but I’m going to take a close look at it. Would be a reasonably easy fix. Thank you.