I am trying to find a "buzzing/humming" noise that occurs after 25mph (40kph), and gets a bit louder as I drive faster.

Heres a list of things i checked:
1) wheel bearings (both replaced within last 3 years) - jacked up both front wheels, shaked them and spun them - no noise and suspension is tight.
2) Front left CV axle replaced 10,000km ago
3) swapped winter tires 2 months ago - with cheaper MotorMaster Winter edge - probably my main culprit
4) front diff fluid hasn't been changed in the last 52k miles (85000km) - not sure if it would get noisy

Car is a 2009 G37x sedan with over 285,000km (175k miles)

the chassis doesn't shake or click. Anyone know what the noise could be?

 

Maybe it is tread noise? I have had some noisy tires, the Pirelli Weather Active sounded like bad wheel bearings (POS tire).

 

What does you tire wear look like? Inner tire camber wear emits a humming sound as Krzysztof47 noted.