Mistery sound.

Hi all,
I just did a rebuild on my engine Cummins N14 Celect Plus, in a '99 Century.
I started getting that weird high pitch sound, when I get up to 10th-11th gear (13 speed) around 1500rpm, the sound stays on for a few seconds, then it goes away on its own. No other changes in the truck going down the road, no vibrations, loss of power, or oil pressure change. I have pulled only empty trailer with it, so don't know if the power is OK when loaded. I took my mechanic for a ride and he seems confused too, doesn't know whee it might be coming from.
Can anyone recognize the origin of this sound, where it may be coming from? I made a video of it, you have to turn on the volume, the sound starts about 5-6 seconds into the clip, then stops, then appears again around 20 seconds.
https://www.youtube.com/shorts/65_cJBqqKwY

 

Thanks for your response. That's what I thought as well, but if it's a boost or exhaust leak, wouldn't the whistling going to be present most of the time and stay on for longer, almost all the time when I am driving in higher gear? That thing stays on for just a few seconds and then goes away.