Low frequency noise while engine braking in 4th and 5th

2009 4cyl Access cab manual transmission with 200k on the truck. New clutch 20,000 miles ago.
When I engine brake mid-range RPMs in 4th and 5th gear I can hear a rather loud vibration/humming sound. There's a low frequency vibration from the engine area (transmission? clutch?). Thinking that maybe the clutch shop put the wrong transmission fluid in, I recently switched to the proper OEM transmission fluid. No change.

Curious if anyone else has this. It started about 5000 miles ago (around 195k miles) and seems to be getting worse. I'm avoiding engine braking of any kind now but would like to know if I should suspect the manual transmission, the clutch, something else, or if it's not something I should worry about.

Any thoughts would be appreciated.