Low moan sound under load

Another question about my 2002 outback 165K miles

When the car is under load i get a loud low pitched moan / groan sound. By under load I mean I press the gas hard to accelerate. Wanted to know if this may be because the engine is older now or not. A few notes / clues on the noise:

  • I didn’t notice it until I changed the spark plugs myself about 1000 miles ago
  • I found cracks in my PCV hose and replaced the hose and valve and found that there was a significant improvement in this noise, but it still persists
  • It does NOT seem to be related to gear. When I quickly press the gas pedal in and out on the freeway for example, I can get the noise to occur (when the pedal is pressed) and go away (when I ease off) without a significant increase in rpm, and without the gear downshifting.
  • It is not simply related to speed or engine RPMs because if I gradually accelerate to a given speed or RPMs then I avoid the noise.

Could this still be vacuum / throttle related? (as I seemed to have an improvement when I changed the PCV hose)

Oh wait… I just had an epiphany… Could this be that I somehow ruined the air intake ducts / resonator (since I removed these when I originally changed the plugs, and I happened to break one of the attachments holding the resonator in because it was rusted) like maybe the plastic is rattling? They are still connected correctly.