A year ago, I replaced both rear axle wheel bearings. A few weeks ago, I've noticed a humming sound that I think is coming from the front of the vehicle. The sound only starts at 55 MPH and does get a bit lower in volume when the speed is increased to 65 MPH.
What could be causing the sound?

 

Hey
oddly enough , the sound could be tyre or bearing. I had humming sound on the back, starting from 55mph like you, sometimes totally no sounds coming.

Tried banking left and right to test if it was bearing. it didn't go off. assuming that it was bearing on the both sides, changed two hub assemblies but its still there . it was the "feather edged" tyre. try Google it

try driving the windows open, and close to a wall, so the sound can echo back. it can be heard even in low speeds with echos. it sounds "chop chop chop"

 

Regarding tires - I also own a VW CC, which has a fairly aggressive toe in to the stock alignment. VW also OEM'd tires to Continental which have a bad reputation for becoming noisy due to feathering (aka cupping). My CC has 45K miles and the tires sound like I'm running snows :-( When I asked around, the recommendation was to 1) check alignment and see if it can be set for the low end of negative toe in and 2) change brand of tire away from OEM Conti's to either their high end or go to another brand entirely. One thing you may want to try is do a visual inspection and see if rotating your tires front to rear changes the noise. If you didn't get a full 4 point alignment done when you replaced the rear bearings, you may want to rotate tires and get the alignment checked. Bad news is if you wind up having to replace the front bearings, you need to get a new alignment done.