^^^^^ Like pa4ul said, the wheel bearing can be a big possibility, My passenger bearing can get loud and have some vibration. Im going on ebay to buy the whole knuckle.

When my filter was clogged, right before I changed it it would do it at any RPM. I changed it right at 104000 (I think). My 04 CVT filter was located under the rear passenger door, right above the plastic cover. you have to be careful with the fuel line clips! They are brittle and break easy!

 

Well my car has 183k and I bought her with 136k and I personally have NOT changed the fuel filter since owning my car but have never heard a hum from the rear passenger side. I frequently drive people around and nobody has said anything about a back seat hum to me.

 

Yeah I have to admit, I was definitely wrong about the placement of the fuel filter. I guess I was thinking about one of my old cars where it was in the top left of the engine bay, the cabin filter system is up there on the Audi's. Not a bad idea to still change the filter I guess, but I don't think it'll solve this issue.

The only reason why I'm questioning the wheel bearing solution is because this sound is almost controllable. I mean, I can give the throttle the 'right' amount of push and it'll make the sound, I let off the throttle and it goes away immediately. It won't be there as soon as I press the throttle again, but I can slow down 5mph or 300-400 rpm and then give the same throttle amount and it'll make the sound again. I've narrowed it down to mostly notice in 2nd gear through 25-35 mph and 6th gear at highway speeds, still between 2200-3300 rpm's.

Is it just me or does that sound way too specific to be a wheel bearing?

 

Does it do it in neutral at 2200-3300 rpm's? I still think its the fuel filter/fuel pump. It would be very noticable in the rear seat when rev'd to 2200-3300. If its while driving, it could be the accesory pullies or tranny related! Good luck!