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Did the dipstick tube pull up out of the pan? It's just in there with orings on the bottom of the tube and the bracket and bolt at the top. Worth a look.

The noise is completely undetectable at engine idle or coasting downhill on the freeway in neutral. If I were to rev the engine, then I might hear a faint hint of it mixed in with the normal engine background noise, but the only thing that makes it loud and vibrate is heavy engine load after being run hard for several minutes. Once the noise becomes prominent, it can still be detected in the background at lower speeds and engine load, but it's hardly noticeable. When it first started, I thought I had hit a rough patch of refinished road that you will sometimes hit on heavily used freeways. That's what it felt like at first, but I soon realized it was an engine issue and not the road or my tires. It's entirely possible that it's something like pre-detonation, low compression, or excessive oil consumption. I didn't have my heads machined when the timing belt snapped in 2015, so compression could be bad, but it's been running 7 years without issue so far. It does certainly consume oil too, but isn't that pretty normal with an old Subaru like this? I thought I noticed it consuming a little more than usual recently, so I will keep an eye on that oil level. It normally consumes about half a quart every 3 fill-ups, so perhaps a quart every 1500 miles? It will consume less with freeway driving; street and heavy traffic driving are what eats a lot of oil for me. I filled up about 50-75 miles before this started happening, at a Shell station. I doubt the gas quality is the issue. What I DID do before this happened was a 200 mile roadtrip and some driving in the town I visited. The noise started on the way back. I actually drove it nearly 200 miles home with that noise, keeping revs high and my gears and speed low to mitigate how bad the sound was getting. I was stopping every one or two freeway exits to check the timing belt and other items under the hood. Once I realized the timing belt wasn't deteriorating, I drove it the rest of the way home, moderating my speed and gears to minimize the noise and vibration. My O2 sensors were both replaced in the past 2-3 years; my catalytic converter is a brand spanking new one from Bosal and I made sure the O2 sensors I used were new. My upstream O2 sensor went bad about 3 years ago, causing me to stall out on the freeway, so I'm familiar with that issue. Both O2 sensors are Denso brand; I see I replaced the upstream O2 sensor first in April 2019. I've never had my valves adjusted. Literally never. Maybe I did it when I had to reassemble the heads after the timing belt snapped, but I don't recall doing so. So I'm guessing you recommend I should get it done. Is that something I could do myself, or is it better to have a shop mechanic do it? When the cam belt snapped there were no significant marks on the face of the pistons. They left cosmetic marks but no substantial indents, and I didn't detect any damage to the cylinder heads either. The valve heads were just bent off to one side at the base of the stem. That's why I felt confident in replacing the valves to fix the issue, and it worked.