Engine humming noise

First post, used this site for a long time but never ran into anything I needed to create a thread for. Thanks for the help in the past.

Been dealing with a strange humming sound recently that increases with the rpms as I accelerate. Once up to speed you can't really hear it anymore, but definitely can when idling and lower rpms. Does it whether it is in gear or not and doesn't increase in loudness when the AC is on or when you turn the steering wheel. I recently removed the serpentine belt, checked all the pulleys to see if I had any play or or strange sounds when spinning by hand. They all seemed fine... put a new belt on as the other had about 195k miles on it (wasn't really in bad shape.. barely any cracks). Drove it for about a week while trying to figure out where the humming was coming from. Well, it ended up throwing the new serpentine belt off and the motor ripped it to shreds. Got it towed back home and ripped the belt out thinking that there has to be pulley frozen up on it now (or obviously bad), still nothing.. they all spin by hand. The worse one was the PS pump so I went ahead and replaced because it had a slow leak anyways. Put the old belt on and it started right up. Still didn't fix my problem. Very hard to pinpoint which side of the motor it is coming from, going to trying using a stethoscope a little later. Any ideas would be greatly appreciated. Thanks!

 

The heat shields on the cats are secure?

 

Likely a bad idler pulley. Cheap and easy enough to change, I would start there while you are already swapping a new belt.

 

Update*
Checked the heat shields, they were secure. Found the main problem sound coming from the alternator using the stethoscope. Also found this post https://www.tacomaworld.com/threads...or-rebuild-mud-cleanout.453757/#post-14645785. The second video sounds exactly like my humming noise. Pulled the electrical connector from the alternator while the truck was running.. humming immediately stopped. Plugged it back in and back came the noise. Going to replace alternator soon.

Not sure why it threw the serpentine belt.. guessing it was either a defective belt or I didn't have it completely lined up with the grooves on one of the pulleys, what I get for doing it at night I guess..