Humming/Whining sound from front end

Hi! I've been a lurker for years but have a problem with my Gen2 that I need to ask about.

A month or ago my wife started complaining about a noise coming from the front end of her Prius.

About the car:
This is a 2008 Prius with 155k miles that spent it's entire life in the rustbelt. I've owned it for 5 years and 30k miles and it's driven regularly. The tires are in good shape, properly inflated, and were balanced a couple of months ago. I changed the brakes and rotors earlier this spring (Power stop kit) and lubed the caliper pins. I change the oil every 5k, had the front bearings replaced 2-3 years ago (KOYO done by a local shop) and changed the inverter coolant pump myself earlier this year. It's in good mechanical condition.

The sound:

• It's coming from the front end.
• You can hear it from inside the passenger compartment but if I roll the windows down and drive by a concrete retaining wall the sound doesn't "reflect" as if it were coming as much from the outside.
• It's dependent on the speed of the car. It starts at about 10mph and gets louder and "higher pitched" as you go faster. It's is very consistent with speed and not related to whether the engine is on or not.
• The sound doesn't seem to change with braking, even when I brake quick and hard.
• The sound doesn't change when turning left or right
• A very good way to describe the sound is like that of a transmission that isn't shifting into a higher gear. Like if you had your car in 2nd gear and didn't shift into 3rd or 4th as you increased speed. It's not a scraping or grinding sound.

Any ideas?