Impreza Vibration on High Speed Breaking

[PA17236]

I have a 2019 Impreza Premium wagon with 86,000 miles, new front brakes and plenty of life in the original rear brakes. When braking moderately at freeway speeds (usually greater than 50 mph, most often above 65 mph), the car makes a low-pitched humming sound and a vibration, which feels like a very fine pulsation, that I feel in the brake pedal. I don't feel it in the steering wheel. The humming sounds like a distant turboprop airplane. The vibration and humming subside as the speed drops below 40 mph. This does not occur at lower speeds, but it's frequent in high-speed heavy traffic requiring a lot of high-speed moderate braking. Hard braking or light braking don't produce the vibration or the sound, so I don't notice it if I have to slam on the brakes. The problem pre-dates the replacement of the front brakes. My mechanic recently checked out the brakes without taking the car on the road and couldn't find anything mechanically wrong with them.

I'm wondering if this is something contaminated brake fluid might cause? Maybe a fault in the antilock braking system?

 

[Ecmbuster]

Find a "smarter mechanic" and have a qualified Technician road test the vehicle at the speeds that are of concern.
Brake fluid exchanges are a good idea during service intervals and testing the ABS is accomplished with a quality scan tool. Cheap off-shore brakes produce less than quality stops at high speeds.
A smart Technician would test the axle bearings and the correct torque. That will include wheel nut torque on all wheels.

 

[sheikh2]

you mentioned changing the brakes, but have you changed the rotors? it is very possible your brake rotors are slightly warped. It's a common thing to replace after 60-90k miles. And cheap too. Under $200-400 considering how long they last.

 

[Ruki]

Your front brake rotors are warped, some shops can turn them on a brake lathe or just replace them.