Loud Hum / Cannot Narrow Down The Issue

[tiffinycostello]

Hey there - I have a 83k mile 2016 Subaru Forester 6-speed manual transmission and I've got a loud rear hum that sounds like an airplane propeller. It seems to be the loudest in 4th gear.

I have checked the wheel bearings, tires, rear differential fluid and for any debris around the tires. Today I checked the transmission fluid and discovered it was down by a quart, however I had it served at Subaru at the 80k mile point, and the hum was happening then, just much quieter. It's very LOUD now.

No error dash codes are coming up on my code reader. I do show my 1st cylinder spark plug misfires every now and then while driving, so I will swap that out but I don't think that is related to this issue.

It hums along with the velocity of the car, not related to the rpm of the engine. Pressing the brakes doesn't disrupt the sound either.

Help! I've got an anomaly on my hands!

 

[Ecmbuster]

Have the vehicle on a hoist, run the engine in 4th gear. Use this tool to listen for the noise.
Low transmission fluid means a leak. Drain all the fluid and look for metal particles.