'04 XS AT - Slight scraping/whirring noise under load?

First-time poster here. My 04 Forester AT with 138k develops a scraping/grinding/whirring noise under load. It seemed to come on pretty sudden a couple months ago and hasn't gone away. It tends to do it once the engine is warmed up, and seems to be coming from the engine compartment. It's not there all the time but seems to be more prevalent on longer trips, especially if I turn off and restart the engine. I've had my AT fluid changed and have gone through 1-2 oil changes since the onset.

Here's what I know:
- It sounds mostly like metal scraping against metal, but can also sound like a belt rubbing against something as well.
- Pitch stays relatively the same but does slightly change with engine RPM, not speed.
- I have not been able to replicate the sound in park or neutral. I have been able to hear it in 2nd, 3rd, & 4th.
- Sound is loudest around 2500-3000RPM and will go away around 4000RPM.
- There's a distinct threshold in the throttle that will "enable" the sound. The sound tends to get louder under more load (i.e. going up a hill) and will go away right before the transmission downshifts.
- The sound goes away while coasting, even at the same RPMs. The sound itself is quiet, unnoticeable if the radio was on.

I've searched through a number of threads trying to find something similar but had no success. Any guesses or at least a way to narrow it down?