Mysterious dry bearing sound near bell housing.

Hi all. I took my 3rd gen in for an oil leak I wasn't interested in fixing myself. It was from a crank seal.
When I got it back, I immediately heard what sounds like a light, dry bearing sound. A shhhh, sound. And it has become louder within the last 400miles.
It only happens when I either put it in drive and apply the breaks, then give it light throttle at around 1200-1500rpm.
Or it happens all the time precisely when I'm at approximately 10 percent throttle and driving down the road.
Sounds like it's coming from the firewall. Kind of where the transmission bell housing is. Initially. I thought it was an intake leak. But the brake parts cleaner spray test didn't reveal anything. Plus there is no check engine light.
I also had put new oil in at the time they had serviced the cam seal. I drained this oil yesterday to inspect, to see if there were any metal residue from potential cam journal wear. If there is any metal wear, it is very, very inconspicuous.
I took it back to the shop and they did not discover anything other than an idler pulley that may have needed replacement. However, that did not solve the problem. So, I took it to two other places and they could not figure it out, but thought it could be a transmission issue.
Then. I took it to a transmission specialist and he seems to think that it is not a transmission issue.
The only thing I can think of, since it has 186k mi. on it, is that it could be a worn out pre-cat. There is some rattling involved from the muffler when I use a rubber mallet.
Any suggestion as to what it could be, is appreciated. I really hate to replace the precat, catalytic converter and muffler, just with suspicions it's the exhaust. I do not see any noticable exhaust leak as well. But either an exhaust leak or a cat that's breaking down inside is my best guess.
Also. While the truck is hot, I measured the inlet and outlet temps of the downstream cat. And found only a 40 degree raise in temps. So.. might this be the likeliest cause?
Thank you very much.